UC-NRLF 


D    E    SE5    bl3 


SMITHSONIAN  CONTRIBUTIONS  TO  KNOWLEDGE. 

1034  - 


Atmospheric   Actino'^.etry 


AND   THE 


ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE. 


E.  DUCLAUX, 

Professor  of  Physics  in  the  Agronomical  Insli/iiU,  Paris. 


CITY  OF  WASHINGTON  : 
PUBLISHED  BY  THE  SMITHSONIAN  INSTITUTION. 

1896. 


SMITHSONIAN  CONTRIBUTIONS  TO  KNOWLEDGE. 
1034  

Ibobghins  jfunb. 


Atmospheric   Actinometry 


AND    THE 


ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE. 


BY 


E.  DUCLAUX, 

Professor  of  Physics  in  the  Agronomical  Institute^  Paris. 


•  •    •  «  •      • 

•  •  •  •  «    «  • 
«  «  ••  •     «• 


CITY  OF  WASHINGTON : 
PUBLISHED  BY  THE  SMITHSONIAN  INSTITUTION. 

1896. 


Ube  'Rntcherbocber  prese,  Dew  ]|2orlt 


ADVERTISEMENT. 


The  present  memoir  is  a  translation  of  the  treatise  entitled  "  Sur 
I'actinom^trie  atmosph^rique  et  sur  la  constitution  actinique  de  I'atmos- 
phere,"  submitted  by  Professor  Emile  Duclaux,  in  competition  for  one  of 
the  Hodgkins  Fund  prizes  offered  by  the  Smithsonian  Institution  in  a 
circular  dated  March  31,  1893.  The  competition  closed  December  31, 
1894;  and  on  August  9,  1895,  the  Award  Committee,  having  completed 
its  examination  of  the  218  papers  submitted  by  contestants,  granted 
honorable  mention  to  Professor  Duclaux  and  recommended  his  memoir 
for  publication  by  the  Smithsonian  Institution. 

The  Committee  was  composed  of  the  following  members :  the  Secre- 
tary of  the  Institution  S.  P.  Langley,  Chairman,  ex-officio :  Doctor  Gr. 
Brown  Goode,  appointed  by  the  Secretary  of  the  Smithsonian  Institu- 
tion ;  Assistant  Surgeon-General  John  S.  Billings,  appointed  by  the 
President  of  the  National  Academy  of  Sciences ;  and  Professor  M.  W. 
Harrington,  appointed  by  the  President  of  the  American  Association  for 
the  Advancement  of  Science.  The  Foreign  Advisory  Committee,  as  first 
constituted,  was  represented  by  Monsieur  J.  Janssen,  Professor  T.  H. 
Huxley,  and  Professor  von  Helmholtz ;  and  after  the  death  of  the  latter, 
Doctor  W.  von  Bezold  was  added. 

S.  P.  LANGLEY, 

SECBETABY. 

Washington  City,  May,  1896. 


1283: 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/atmosphericactinOOduclrich 


Atmospheric  Actinometry  and  the  Actinic  Constitution 

OF  THE  Atmosphere. 


By  E.  Duclaux, 

Professor  of  Physics  in  the  Agronomical  Institute^  Paris. 


ATMOSPHERIC  ACTINOMETRY. 

The  progress  made  by  science  leads  us  more  and  more  to  attribute  to  chemical 
rays  a  special  action,  which  is  different  from  and,  to  a  certain  extent,  independent 
of  that  of  the  calorific  and  luminous  rays.  The  chemical  radiations  of  the  sun, 
reaching  the  limits  of  our  atmosphere,  become  modified  while  passing  through  it, 
according  to  a  law  which  is  peculiar  to  them  ;  and,  so  far  as  can  be  seen  in  so  new 
a  subject,  their  absorption  is  not  the  same  as  that  of  the  calorific  or  luminous  parts 
of  the  spectrum. 

Photographers,  especially  those  who  take  landscapes,  well  know  that  days 
which  are  equally  warm  or  equally  luminous  do  not  always  give  the  same  results 
for  the  same  length  of  exposure,  and  that  there  are  days  when,  for  some  unknown 
reason,  the  chemical  impression  is  much  slower  than  on  others. 

Another  argument  may  be  drawn  from  what  often  happens  in  northern  lands, 
where  vegetation,  which  is  well  known  to  be  specially  susceptible  to  the  power  of 
chemical  rays,  makes  much  more  rapid  progress  than  in  temperate  I'egions,  notwith- 
standing the  fainter  light  and  the  lower  temperature. 

To  what  are  such  differences  due  ?  What  law  does  the  chemical  absorption 
of  the  atmosphere  obey,  and  on  what  does  it  depend  ?  Ought  we  to  attribute  it  to 
its  normal  elements:  oxygen,  nitrogen,  carbonic  acid,  and  water  vapor?  Then  it 
should  have  some  general  uniformity.  Or  ought  we  to  see  in  it,  on  the  con- 
trary, the  action  of  solid  or  volatile  elements,  which  incessantly  reach  it  from  the 
bare  or  from  the  cultivated  soil  ?  Then  it  should  have  a  local  character,  leading 
to  a  multiplicity  of  chemical  climates.     These  are  very  importatft  questions,  for 


2  ATMOSPHERIC  ACTINOMETRY 

which  science  has  as  yet  no  answer ;  not  that  the  subject  has  not  already  been 
thoroughly  investigated,  but  because  in  all  the  actinometric  inquiries  proposed  so 
fai',  sufficient  care  has  not  been  bestowed  upon  the  separation  of  chemical  action 
from  luminous  and  calorific  effects. 

The  process  which  best  shows  the  incorrectness  of  the  methods  employed 
heretofore  is  that  of  Messi's.  Bunsen  and  Roscoe,  which  depends  upon  a  mixture 
of  chlorine  and  of  hydrogen,  exposed  to  the  light.  The  intensity  of  the  chemical 
action  is  then  estimated  by  the  quantity  of  hydrochloric  acid  formed  in  a  given  time, 
or  rather  by  the  diminution  of  volume  which  necessarily  follows.  This  method 
has  two  grave  defects.  One  is  that  reaction  may  take  place  from  the  effect  of  heat 
quite  as  well  as  from  that  of  chemical  rays,  and  that  consequently  it  does  not 
separate  the  two  actions  which  it  is  important  to  isolate.  The  second,  much  more 
sei'ious,  defect  is  this,  that  the  reaction  is  extremely  exothermic  and  continues, 
when  once  begun,  under  the  influence  of  the  heat  which  it  develops.  There  is, 
therefore,  no  proportionality  between  the  active  cause  and  the  effect  it  produces. 
The  cause  is  simply  provocative  and  starts  a  mechanism,  which  continues  to  work 
independently.  It  is  true  that  an  effort  is  made  to  reduce  to  a  minimum  the  work 
of  this  mechanism,  by  operating  only  with  very  small  quantities  of  gas  and  by 
multiplying  the  cooling  surfaces,  in  such  a  way  that  the  phenomenon  constantly 
requires  a  new  excitation  in  order  to  continue.  But  this  is  not  suflScient  to  relieve 
the  method  of  the  charge  of  lacking  proportionality  between  cause  and  effect,  which 
renders  the  measurements  almost  illusoiy,  in  spite  of  the  care  taken  by  Messrs. 
Bunsen  and  Roscoe  to  discuss  them. 

We  find  the  same  defects,  though  perhaps  a  little  less  seriously,  in  the  often 
employed  method  which  depends  upon  the  reduction  of  ferric  oxalate  by  light. 
Since  the  first  observation  by  Dobereiner,  H.  Draper,  Marchand,  and  G.  Lemoine 
have  studied  this  reaction.  As  in  the  preceding  case  the  oxalate,  or  the 
equivalent  mixture  of  feri'ic  chloride  and  oxalic  acid,  is  reduced  by  the  action  of 
heat  alone,  and  although  this  reduction  is  slow,  it  operates  as  a  source  of  error. 
Moreover,  the  liquid  is  colored  and  loses  its  color  in  proportion  as  the  pi'ocess 
continues.  Hence  the  conditions  of  absorption  are  modified  during  the  process 
and  this  by  a  phenomenon  which  is  to  a  certain  degree  external.  Finally,  the  re- 
action is  still  sufficiently  exothermic  to  require  that  this  property  should  be  taken 
into  account.  All  these  defects  have  been  corrected,  so  far  as  possible,  by  M.  G. 
Lemoine,  who  has  for  some  time  been  making  a  careful  study  of  the  process,  but 
the  method  loses  thus  that  neatness  and  that  simplicity  w^hich  are  so  desirable. 

The  ideal  would  be  attained  by  the  discoveiy  of  a  limpid  and  transparent 
liquid  which  would  not  change  while  the  reaction  went  on,  becoming  the  seat  of 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  3 

an  easily  measurable,  chemical  phenomenon,  which  could  be  the  result  of  no  action 
except  that  of  heat.  Can  we  go  any  faither  in  our  demands  and  require  that  it 
should  not  be  exothermic  in  any  degree  ?  M.  Berthelot  does  not  think  so,  and  be- 
lieves that  the  addition  of  enei'gy  resulting  from  the  absoi'ption  of  calorific,  lumi- 
nous, or  chemical  radiation  would  not  be  sufficient  to  produce  a  chemical  phenomenon 
which,  while  going  on,  would  not  give  rise  to  a  small  amount  of  heat.  .  I  do  not 
very  well  see  why  a  calorific  absorption  of  solar  radiation  might  not  compensate 
for  some  heat  of  combination,  and  even  permit  a  slightly  endothermic  reaction  to 
appeal'.  I  have  endeavored  to  discover  some  simple  method,  which,  complying 
with  this  programme,  could  be  interpreted  without  ambiguity,  but  I  have  not 
succeeded.  I  have  been  compelled  to  content  myself  with  an  old,  well  known 
reaction — the  oxidation  which  weak  solutions  of  oxalic  acid  undergo  upon  exposure 
to  light. 

These  solutions  are  and  remain  transparent.  The  oxalic  acid  in  them  is  ti-ans- 
formed  into  carbonic  acid,  which  disappears  by  diffusion,  so  that  the  oxidation 
which  it  has  undergone  can  be  easily  ascertained  by  an  acidimetric  determination- 
made  before  and  after  its  exposure  to  light.  The  reaction  thus  produced  is  faintly 
exothermic,  to  be  sure,  but  as  only  very  weak  solutions  are  taken,  for  it  is  well  not 
to  exceed  2  or  3  grammes  of  crystallized  oxalic  acid  per  litre,  thei-e  is  no  reason 
why  we  should  be  troubled  about  the  error  which  arises  from  this  fact.  Moreover, 
the  liberation  of  heat,  which  results  from  combustion,  even  if  it  should  be  percep- 
tible, would  i-emain  without  effect,  for  oxalic  acid  oxidizes  onl}^  with  extreme  slow- 
ness under  the  influence  of  heat  alone. 

Exp. — 10  c.  c.  of  a  solution  of  oxalic  acid,  titrating  19  c.  c.  of  lime-water  per  litre, 

was  heated  on  a  water  bath  to  nearly  95°,  in  a  flask  of  125  c.  c. 

After  heating  4  hours,  the  titre  is  18.5  c.  c.     Loss  2.6  per  cent. 
"  "         8      "         "         "       18.0     "  "      5.2         " 

Exp. — 10  c.  c.  of  another  solution,  titrating  16.6  c.  c.  of  lime-water  per  litre,  was 

heated  an  hour  and  a  half  to  115°. 

The  titre  falls  only  to  16.2  c.  c.  and  16.1  c.  c. 
During  the  days  of  greatest  heat,  the  temperature  hardly  exceeds  50°  in  the 
shallow  vessels,  in  which  the  liquid  is  exposed  to  the  sun.  It  may,  therefore,  be 
assumed  that  neither  the  solar  heat  nor  the  heat  produced  by  combustion  have  any 
percej)tible  effect  upon  the  transformation  of  the  oxalic  acid,  which  may,  on  fine 
days,  reach  or  even  exceed  50  per  cent  of  the  acid  contained  in  the  solution. 

As  the  calorific  rays  have  hardly  any  power,  it  would  be  desirable  to  eliminate 
the  action  of  the  luminous  rays  also,  but  the  chemical  radiations  are  so  closely  in- 
termingled with  the  latter  that  it  is  difficult  to  sepai-ate  them.     Let  us  be  content, 


4  -  ATMOSPHERIC  ACTINOMETRY 

therefore,  for  tlie  present,  to  know  that  our  solution  of  oxalic  acid  is  peculiarly 
affected  by  the  action  of  the  luminous  and  the  chemical  parts  of  the  spectrum. 
We  shall  soon  find  reasons  to  believe  that  it  is  the  chemical  part  alone  which  acts. 
But  we  have  first  to  investigate  the  manner  by  which  combustion  is  produced 
before  we  can  determine  what  influences  cause  it. 

Study  of  the  Pkocess. 

Oxalic  acid,  dissolved  in  water  and  exposed  to  light,  absorbs  oxygen,  and 
changes  almost  entirely  into  carbonic  acid.  There  appears  also  a  little  formic  acid, 
but  in  almost  infinitesimal  quantities.  Hence  it  follows,  as  we  have  seen,  that  we 
can  ascei'tain  the  quantity  of  oxidized  acid  by  a  simple  acidimetric  detei-mination. 

INl'LUENCE  OF  CONCENTRATION. 

In  order  to  study  the  actinometric  process,  the  first  thing  to  discover  is  the 
degree  of  concentration  which  gives  the  lai'gest  amount  of  sensibility.  In  order  to 
know  this  I  exposed  to  the  sun,  under  precisely  the  same  conditions,  during  three 
fine  days  from  June  4th  to  June  6th,  including  about  36  houi's  of  insolation,  four 
liquids,  containing,  respectively,  per  litre  : 


^rm. 

grin. 

grm. 

grm. 

63 

31-5 

12.6 

6.3  of  oxalic  acid  ; 

I 

4 

i 

t'^  of  an  equivalent, 

that  is, 

per  litre.     At  the  end  of  this  time,  an  acidimetric  analysis  gave  nie  the  quantities 

of  acid  which  had  been  burnt,  and  I  computed  from  this  the  piopoi-tion  of  acid 

which  had  disappeared  from  each  of  the  vessels. 

The  figures  wei-e  the  following,  counted  in  milligrammes  per  litre : 

1  Equiv.  I  Equiv.  ^  Equiv.  -jV  Equiv. 

Quantity  of  acid  burnt,    2,500  2,800  4,700  3,300 

Proportion,  4^  9^  38^  52^ 

To  reach  the  maximum  in  the  absolute  quantity  of  acid  consumed  we  nuist, 
therefoi-e,  operate  with  solutions  neither  too  concentrated  nor  too  weak.  Solutions 
which  are  too  concentrated  oxidize  slowly,  and  the  variation  in  chemical  value  is 
often  noted  with  difficulty.  It  is,  on  the  other  hand,  very  easy  to  measure  this 
variation  with  solutions  which  are  rather  weak,  because  it  represents  a  notable 
fraction  of  the  primitive  value.  But,  on  the  other  hand,  when  the  liquid  has  been 
weakened  by  the  sun,  the  last  portions  burn  quite  slowly.  There  aie,  therefore, 
two  dangers  to  avoid. 

After  various  trials  I  decided  upon  a  solution  whose  variation  of  titre  during 
the  most  favorable  days  should  not  exceed  one  half  of  the  initial  value.     This  is  a 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  5 

solution  containing  about  ^  an  equivalent  or  about  3  grammes  of  oxalic  acid  per 
litre.  Ten  cubic  centimetres  of  this  liquid  are  saturated  by  about  an  equal  volume 
of  common  lime-water,  so  that  the  daily  variation  of  the  titre  amounts  to  4  or  5  c.  c. 
of  lime-water,  a  quantity  which  can  be  measured  down  to  j^^  by  means  of  a 
burette.  The  accuracy  which  we  thus  obtain  is  moi'e  than  sufficient,  as  we  shall 
presently  see. 

rNFLUENCE   OF   THE   DEPTH    OF    SOLUTION. 

In  discussing  the  question  of  oxidation,  we  must  consider  the  part  played  by 
the  ease  with  which  oxygen  penetrates  into  the  depths  of  the  solution.  We  can 
easily  calculate  that  the  10  c.  c.  of  oxalic  solution,  which  are  used  in  each  one  of 
the  experiments,  require  for  complete  combustion  about  3  c.  c.  of  oxygen,  a  quantity 
greatly  superior  to  that  which  is  alieady  disvsolved.  Whatever  the  facility  may  be 
with  which  this  gas  penetrates  into  a  liquid,  which  is  subjected  for  8  to  10  hours 
to  insolation  in  free  contact  with  the  air,  we  may  well  ask  if  a  solution  of  oxalic 
acid  oxidizes  in  the  sun  in  the  same  way  in  a  vessel  of  shallow  depth,  in  which  its 
thickness  is  small,  as  in  a  cone-shaped  glass  or  in  a  round  tube.  The  following 
experiment  fui-nishes  an  answer  to  this  question  : 

Mep. — On  August   16th,  17th,  and  18th,  I  exposed  to  the  sun  10  c.  c.  of  a  -^\- 
normal  solution  of  oxalic  acid,  as  follows  : 

(a)  In  a  cone-shaped  glass, 

(b)  In  an  ordinary  test  tube, 

(c)  In  a  Bohemian  glass  matti'ass  with  flat  bottom. 

To  secure  uniformity  of  tempei'ature,  the  cylindrical  tube  b  was  placed 
upright  in  the  mattrass  c ;  the  exposure  continued  from  8  o'clock  a.m.  till 
3.30  in  the  evening.     The  following  proportions  of  acid  were  consumed  : 


a 

b 

c 

August  i6th, 

29^ 

-% 

65 

"     lyth, 

34 

14 

97 

"       1 8th, 

34 

'3 

84 

"       19th, 

31 

14 

87 

Thus,  eveiything  else  being  equal,  the  proportion  of  acid  consumed  is  nuich 
greater  in  a  vessel  with  a  flat  bottom  than  in  a  cylindiical  tube.  The  difference  is 
indeed  so  very  striking,  that  the  difficulty  with  which  oxygen  penetrates  the  solu- 
tion does  not  suffice  to  explain  it. 

A  combustion  of  13  per  cent,  produced  in  7  hours  in  the  10  c.  c.  of  liquid, 
contained  in  the  mattrass  b,  has  not  required  more  than  0.4  c.  c.  of  oxygen  ;  in 
other  words,  about  6  times  the  normal  quantity  dissolved  in  the  solution.     When 


6  ATMOSPHERIC  ACTINOMETRY 

we  think  of  the  rapidity  witli  vvhicli  de-aerated  water  aerates  itself  anew,  it  is  hard 
to  believe  that  it  was  the  oxygen  wliich  was  wanting,  and  we  are  thus  led  to  be- 
lieve that  the  chemical  action  was  at  fault.  If  the  incident  ray  does  not  bring  with 
it  an  excess  of  chemical  enei'gy,  the  superficial  la3^ers  absorb  as  much  as  is  available, 
and  thei'e  is  none  left  for  the  lower  strata,  even  though  all  the  needful  oxygen 
should  be  at  hand  to  burn  the  acid  which  is  present. 

The  question  is  of  some  importance,  because  it  teaches  us  the  quantum  of 
chemical  action  which  may  be  expected  from  light  in  the  vicinity  of  the  soil,  and 
consequently,  also,  the  degree  of  atmospheric  absorption.  In  oi'der  to  get  informa- 
tion on  this  subject,  let  us  operate  with  shallow,  cylindrical  vessels,  which  are  at 
most  a  centimetre  high  at  the  rim,  so  that  there  can  be  no  stagnation  of  air  above  the 
liqiiid,  and  that  the  oxygen  always  has  easy  access  to  the  latter.  If  the  actinic 
influence  is  deficient  in  the  incident  light,  we  must  be  able  to  put  in  evidence  the 
influence  of  the  surface  and  the  depth  of  the  liquid.  For  equal  depths  the  com- 
bustion will  have  to  be  propoi-tional  to  the  surface.  Foi'  equal  surfaces  with  dif- 
ferent depths,  combustion,  if  limited  to  the  superficial  layeis,  should  not  increase 
with  the  volume  and  the  depth  of  the  solution,  or  at  least  not  inci-ease  so  rapidly. 
This  is  exactly  what  experience  shows. 

Exp. — ^Into  two  cylindrical,  very  shallow  vessels,  having  the  same  surface,  I  poured 
10  and  20  c.  c.  of  a  half-deci-normal  solution  of  oxalic  acid.  After  a  rather 
dark  and  somewhat  stormy  day,  I  find  that  28  per  cent,  of  the  acid  has 
been  burnt  in  the  vessel  that  held  10  c.  c,  and  only  23  per  cent,  in  the  other. 
As  it  held  twice  as  much  liquid  as  the  other,  the  absolute  quantities  of 
acid  burnt  are  relatively  28  and  46,  while  the  depths  of  solution  were  in 
the  ratio  of  1  to  2.  Combustion,  therefore,  increases  less  quickly  than 
depth.  As  the  latter  has  not  exceeded  a  centimetre  in  the  vessel  in 
which  it  was  greatest,  and  as,  moreover,  the  total  combustion  was  very 
slight,  we  cannot  admit  that  oxygen  was  wanting.  But  the  solar  rays, 
deprived  of  their  chemical  radiations,  which  were  rendered  active  by  their 
passage  thiough  the  superficial  strata,  reached  the  lower  layers  very  nuich 
weakened,  although  the  luminous  transparency  of  the  two  liquids  was 

perfect. 
There  exists  then  a  kind  of  shifting  of  the  actinic  rays  during  the  passage  of 
the  light  through  the  first  layers  which  it  encounters;  and,  whether  these  rays  are 
not  abundant  or  whether  the  absorption  be  very  efficient  and  the  medium  very 
opaque  for  them,  the  weakening  process  is  very  rapid.  In  return,  when  the  surface 
alone  is  allowed  to  vary,  while  the  height  of  the  liquid  remains  unchanged,  the 
effect  of  combustion  is  proportional  to  the  surface,  and  consequently  to  the  volume. 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  7 

Myp. — I  procured  two  cylindrical  vessels  with  flat  bottoms,  of  Bohemian  glass, 
the  bottom  surfaces  of  which  were  as  1  to  2.     I  exposed  them  to  the  sun, 
one  with  10  c,  c,  the  other  with  20  c.  c.  of  one  and  the  same  half-deci-norraal 
solution  of  oxalic  acid.     The  quantities  of  burnt  acid  have  always  been  in 
the  propoi-tion  of  1  to  2,  in  a  long  sei-ies  of  experiments,  with  an  approxi- 
mation equal  to  that  which  the  process  of  analysis  should  demand. 
We  shall  have  to  avail  ourselves  of  all  these  results  when  we  try  to  ascertain 
the  cause  of  atmospheric  absorption.     Let  us  be  content,  for  the  time  being,  with 
drawing  a  practical  conclusion  from  them,  namely,  that  it  is  desirable  always  to 
work  with  vessels  of  the  same  dimensions,  and  with  e<pial  quantities  of  solution,  if 
we  wish  to  obtain  figures  that  can  be  compared  with  each  other. 

I  employ  small  blown  vessels  of  Bohemian  glass,  with  flat  bottoms,  such  as  are 
found  in  trade.  I  choose  tliem  of  the  same  dimensions,  or  nearly  so,  which  can 
easily  be  ascertained  by  fitting  together  their  edges  and  noting  whether  they  have 
nearly  the  same  external  diameter.  It  is  not  necessary  to  carry  accuracy  any 
fai-ther,  considering  all  the  inevitable  irregularities  connected  with  measurement. 

Those  which  I  have  used  measured  about  4.5  centimetres  in  diaineter,  and 
10  c.  c.  of  liquid  had  there  a  thickness  of  about  6  millimetres.  When  I  was  at  work 
in  the  country,  in  the  Cantal  or  in  the  Puy-de-Dome,  where  the  clouds  of  atmos- 
pheric dust  are  not  calcareous,  I  left  them  freely  exposed  to  the  air  upon  a  small 
table,  so  placed  as  to  face  the  south  and  to  expose  them  to  the  sun  all  day  long. 
The  heating  which  takes  place  in  them  is  never  very  great,  as  the  following 
experiments  show  ;  although  they  were  not  made  in  a  flat  vessel,  but  in  a  cone- 
shaped  glass  with  a  foot.  The  heating  is  less  by  4°  or  5°  C.  when  working  with 
a  flat  vessel. 

Mi^. — The  same  glass  with  a  foot  and  containing  10  c.  c.  of  a  half-deci-normal 
solution  of  oxalic  acid  was  exposed  from  the  15th  to  the  27th  of  August, 
1885,  daily  to  the  sun.  Every  day  the  mean  pressure,  the  maximum 
temperature  of  the  liquid,  and  the  aspect  of  the  sky  were  carefully  noted. 
Here  follow  the  proportions  of  acid  burnt  on  the  different  days,  during 
which  the  weather  was  very  fine.  The  experiments  were  made  at  Fau,  in 
the  Cantal,  at  an  altitude  of  about  700  metres. 


ATMOSPHERIC  ACTINOMETRY 


Date. 

Barom. 

Pressure. 

Max.  Temper. 

Combustion. 

Condition  of  the  Sky. 

August   15. 

712 

mm. 

36°.  8 

21  i 

Fine.     Slight  cirrus. 

"        16. 

712 

t< 

37°-S 

29  i 

Fine. 

'T- 

710 

37°.5 

34  i 

IS. 

710 

37°-S 

32^ 

"        19- 

70s 

3S°-9 

32  i> 

East  wind.     Clear  sky. 

"            2C. 

705 

3S''.2 

30  i 

t<                tt                        U            (( 

"            21. 

70s 

30° 

24  i 

Cirro-cumulus.     Fresh  weather. 

"            22. 

70s 

25° 

24  ^ 

Sky  clouded  in  the  morning. 

"            23- 

705 

22° 

22  <f, 

Cumulus. 

24. 

710 

0 

30 

27  i 

Sky  overcast. 

"            25. 

710 

2S° 

25  i 

Stormy  weather.     Rain  at  night. 

26. 

705 

30° 

30  i 

Cumulus. 

"           27- 

700 

29" 

26  ^ 

The  most  active  combustions  correspond  to  the  highest  maximum  temperatures, 
but  only  because  both  of  them  indicate,  each  in  its  own  way,  the  presence  of  a 
livelier  and  more  active  sunlight.  When  the  sky  is  overcast  or  shows  cumuli, 
the  solar  combustion  may  be  more  powerful  than  when  there  are  cirri,  even  though 
the  maximum  temperature  should  be  lower. 

In  no  case,  as  will  be  seen,  has  the  temperature  of  the  solution  risen  to  a  suffi- 
ciently high  level  to  affect  the  chemical  combustion  which  takes  place  there.  We 
may,  however,  if  we  wish  it,  secure  ourselves  against  this  cause  of  eri'or  by  causing 
the  light  vessels  which  contain  the  solution  of  oxalic  acid,  to  float  on  a  water-bath. 
They  will  then,  during  the  day  on  which  they  are  exposed  to  the  sun,  be  heated  a 
few  degrees  only.  This  is  a  method  which  I  have  adopted  only  during  the  hottest 
and  driest  days.  The  water-bath  served  as  much  to  resti'ain  evaporation  as  to 
prevent  heating  of  the  solution. 


INFLUENCE   OF   THE   AGE    OF   THE    SOLUTION. 

We  now  reach  an  unexpected  fact,  namely :  that  a  fresh  solution  of  oxalic 
acid  does  not  behave  like  an  older  solution  of  the  same  strength,  and  appears  much 
more  refractory  to  the  action  of  the  sun.     It  becomes  sensitive  only  very  slowly, 
and  it  requires  even  several  weeks  for  that  end,  when  it  is  kept  in  diffused  light. 
Etcp. — On   September  5,  1885,  I  compared  an  old  solution  of  oxalic  acid,  con- 
taining -^  equivalent  (1.575  grm.)  of   this  acid  per  litre,  with  another 
liquid,  which  I  prepared  at  that  moment,  of  the  same  sti'ength.     The  com- 
mon titre  of  these  two  solutions  amounted  to   22.8   c.  c.  of  lime-water 
for  20  cubic  centimetres. 

At  the  close  of  the  day   (September  5th),  which   had  been  rather 
foggy,  two  insolated  vessels  containing  the  older  liquid  titrated   togethei- 


AND  THE   ACTINIC  CONSTITUTION    OF  THE  ATMOSPHERE.  9 

16.2  c.  c.  of  the  same  lime-water  and  had  consequently  lost  6.6  c.  c.  of 
tlieir  original  strength.  Two  vessels  with  the  new  solution  titrated  to- 
gether 21.7  c.  c.  and  had  consequently  lost  not  more  than  1.1  c.  c.  The 
new  solution  is  therefore  nearly  6  times  less  sensitive  than  the  other. 

The  next  day,  the  weather  being  fine,  the  losses  amounted  to  1.5  c.  c. 
for  the  new  solution  and  to  8.5  c.  c.  for  the  old.  This  is  about  the  same 
ratio  as  on  the  day  before. 

On  September  12th,  after  a  fine  day,  four  trials  gave  me  the  same 
results,  losses  of  9.3  c.  c.  for  the  old  solution,  and  of  5.9  c.  c.  foi-  the  newly 
made.  The  difference  in  sensitiveness  was  less  marked  than  six  days 
previously. 

On  September  25th,  twenty  days  latei-,  the  losses  became  8.6  c.  c.  for 

the  first  liquid,  and  7.7  c.  c.  for  the  second.     This  is  not  yet  equality, 

which  was  reached  only  in  the  month  of  October,  after  a  little  more  than 

a  month. 

The  fact  that  two  liquids  of  different  ages  reach  at  the  end  of  some  time  the 

same  degree  of  sensitiveness,  proves  that  thei-e  must  be  a  maximum.     We  shall, 

however,  soon  see  that  this  is  not  a  maximum  mcuximcyrnm.     However  this  may  be, 

if  the  solution  of  oxalic  acid  has  once  reached  this  maximum,  it  differs  in  no  way 

from   what  it  was  at  first,  neither  from  a  chemical  nor  from  a  physical  point  of 

view  ;  it  gives  by  evaporation  the  same  crystallized  acid,  and  its  acidiraetric  value 

is  unchanged.     A  molecular  activity,  however,  has  been   at   work,  upon  which  I 

shall  not  dwell  just  here.     I  will  state  now  only  two  important  facts  concerning  it: 

one  is  that  it  requires  time  for  its  completion,  and  the  other  that  it  betrays  itself 

by  easier  oxidizability  under  the  influence  of  solar  radiations. 

The  only  phenomenon  which  in  our  present  state  of  knowledge  may  be  com- 
pared to  tliat  which  we  have  just  discovered,  is  the  increase  of  sensitiveness 
observed  in  sensitized  collodion  which  has  been  allowed  to  rest  and  to  grow  old 
for  a  few  days.  This  fact  is  well  known  to  photographers.  It  may  make  us  think 
also,  by  analogy,  of  the  valuations  in  the  rotatory  power  of  sugar  solutions,  some 
hours  after  their  preparation  up  to  the  moment  when  they  become  stable.  It  is 
admitted  that  these  few  hours  are  necessary  to  enable  the  sugar  molecules  to  spread 
uniformly  throughout  the  solution  and  to  assume  the  orientation  necessary  to 
stable  equilibrium.  But  all  these  analogies  are  remote.  The  phenomenon  deserves 
being  investigated  by  itself,  and  we  have  here  only  to  face  its  practical  conse- 
quences. 

These  may  be  summed  up  in  a  few  words :  that  it  is  desirable  to  allow  the  oxalic 
acid  solution  to  acquire  such  sensitiveness  before  using.     This  is  all  the  easier  since 


10  ATMOSPHERIC  ACTINOMETRY 

these  solutions  can  become  sensitive  in  a  concentrated  state  and  preserve  this  sensi- 
tiveness even  after  being  diluted.  One  can  then  provide  a  mother-liquor,  so  to 
speak,  which  may  be  made  sensitive  and  w^hich  afterwards  may  be  diluted  as 
necessity  arises.  Ordinarily  I  used  to  prepare  a  normal  solution  to  the  amount  of 
several  litres,  containing  13  grammes  per  litre,  which  I  kept  for  some  weeks  under 
diffused  light,  and  subsequently  diluted,  in  fractions,  to  the  twentieth  or  fortieth 
degree.  One  litre  of  this  mothei'-liquid,  rendered  duly  sensitive,  may  thus  serve 
for  2000  tests.  In  all  the  comparative  experiments  which  will  be  mentioned  in 
this  memoir,  I  have  always  taken  pains  to  work  with  identical  liquids  and  such  as 
had  the  same  sensitiveness. 

We  are  now  possessed  of  oui"  actual  working  process,  which  amounts  to 
this :  To  expose  to  the  sun  during  the  day  a  shallow  dish,  containing  20  c.  c.  of  a 
half-deci-normal  solution  of  oxalic  acid,  which  has  become  sensitive  by  time,  and  to 
measure  at  the  close  of  the  day,  by  a  titration  with  lime-water,  the  quantity  of  acid 
which  has  disappeared  by  oxidation. 

Let  us  now  see  what  results  have  been  obtained  by  this  process. 

ACTINOMETRIC     MEASUREMENTS. 

Since  the  year  1885  I  have  made  several  series  of  actinometric  measurements, 
especially  dui'ing  fine  weather  and  at  times  when  I  was  sufficiently  master  of  my 
own  time  to  secure  to  them  the  regularity  which  they  require.  All  these  experi- 
ments, made  at  different  times  and  at  different  places,  are  not  absolutely  alike,  since 
the  solutions  used  might  have  undergone  some  change.  But  such  variations  amount 
to  little  from  one  year  to  another,  and  to  almost  nothing  in  the  course  of  the 
same  year,  as  I  have  been  able  to  determine  repeatedly ;  for  every  time  when  I 
changed  the  solution,  I  exposed  simultaneously  two  or  more  vessels  with  old  and 
with  new  material,  and  I  always  found  that  the  solar  combustion  was  the  same  for 
both,  up  to  that  degree  of  approximation  which  the  measurements  demand. 

While  operating  with  two  or  more  vessels  containing  one  and  the  same  liquid, 
it  does  not  always  happen  that  we  find  the  same  result  for  all  at  the  close  of  the 
day.  There  are  irregularities  in  the  process,  some  of  which  will  be  explained 
presently,  while  the  others  have  until  now  defied  all  efforts  at  explanation,  so  sud- 
den are  they  and  so  exceptional.  There  is  no  other  remedy  for  this  than  to  elimi- 
nate such  out-of-the-way  cases,  which  are  always  rare,  making  every  day  a  trial  with 
3  or  4  vessels  and  keeping,  of  the  figures  thus  obtained,  only  those  which  are  con- 
cordant. 

It  is  in  this  way  that  the  following  observations  have  been  made.  For  each  of 
them  a  record  has  been  kept  of  the  proportion  of  oxalic  acid  burned  in  10  c.  c.  of  a 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  H 

half-deci-normal  solution  exposed  to  the  sun.  Fuitliermore  the  state  of  the  sky  has 
been  recorded  and  the  principal  incidents  of  the  day  of  insolation. 

Special  attention  has  been  paid  to  verifying  the  solar  and  antisolar  lights ' 
which  were  very  fi-equent  during  the  first  years  in  which  these  observations  were 
made  and  which  have  never  been  absent  since  that  time.  I  have  desciibed  the 
aspect  they  assumed  in  the  countries  in  which  I  began  my  observations.  The  more 
1  study  them,  the  more  I  consider  them  as  solely  due  to  the  presence  of  aqueous 
vapor  at  very  great  heights  in  the  atmosphere.  We  shall  have  to  examine,  from 
this  point  of  view,  their  influence  on  the  phenomena  of  solar  combustion. 

The  tables  which  follow  are  also  intended  to  show  the  very  considerable 
vai'iation  which  the  quantities  of  oxalic  acid  consumed  present  from  day  to  day. 
The  combustion,  which  is  almost  completely  absent  on  cloudy  or  rainy  days,  may 
reach  or  even  exceed  50  per  cent  of  the  acid  during  bright  and  luminous  days.  But 
there  are  also  some  very  bright  days,  during  which  combustion  is  feeble,  and  twice 
it  has  happened  that  I  was  unable  to  take  photographs  for  want  of  proper  light, 
being  deceived  by  the  apparent  biightness  of  the  day  on  which  I  was  working. 

I  shall  quote  ray  observations  very  nearly  in  the  order  in  which  I  made  them, 
from  the  moment  when  I  had  legulated  the  process  of  measurement ;  and  in  con- 
nection with  each  one  of  these  sets  of  observations  I  shall  cite  the  facts  which 
they  have  revealed  to  me  and  which  subsequent  observations  have  only  confirmed. 

OBSERVATIONS  OF  THE  YEAR  1885. 

Made  at  Fau  (Cantal).  Altitude  800  metres.  Country  of  meadows  and  of 
woods.     Volcanic  soil.     (Andesite  and  basalt  of  the  plateau.) 

I  have  inserted  above  (page  8)  some  observations  which  I  made  at  the  end  of 
August  with  the  solution  in  a  conical  glass.  The  combustion  is  a  little  less  rapid 
than  in  the  vessels  of  Bohemian  glass  which  I  used  in  experiments  of  a  later  date. 
Here  follow  those  made  in  September  and  October.  S.  and  A.  S.  repiesent  solar 
lights  in  the  west  and  anti-solar  lights. 

'  See,  on  this  subject,  a  note  inserted  in  W\e.Comptes  Rendus  de  I' Academie  des  Sciences,  yo\. 
xcix,  p.  714. 


12 


ATMOSPHERIC   ACTINOMETRY 


Date. 

Combustion. 

Remarks. 

September  2 

32^ 

Rain  the  night  before.     Clear  weather.     S.  faint. 

a 

3 

0^ 

Rainy  and  stormy  day.     No  suft. 

a 

4 

li 

Glimpses  of  the  sun.     Barometer  rises  again. 

(( 

S 

13^ 

Day  jjartly  sunny,  partly  rainy. 

C( 

6 

7^ 

Rain  in  the  morning  ;  a  little  sun  in  the  evening. 

(( 

7 
8 

7^ 
28^ 

t(        ((       ^^               ((                  »<       a          i*.       (t       <t             ti 

u 

Day  in  appearance  similar  to  the  two  preceding. 

it 

9 

Ti 

Fog  and  rain  in  the  morning  ;  sun  in  the  evening.  S.  and  A.  S.  con- 
tinuous during  more  than  an  hour. 

it 

10 

35^ 

Very  fine  day.     S.  and  A.  S. 

ti 

II 

28^ 

Rainy  in  the  morning.  Clearing  in  the  evening.  S.  and  A.  S.  very 
bright. 

i( 

12 

36^ 

Fine  day,  corona  around  the  moon.     S.  and  A.  S. 

if 

13 

22  <^ 

A  fine  day.  Cirrus.  In  the  evening,  at  sunset  clouds  in  the  west 
project  their  shadow  in  the  east  upon  anti-solar  lights.  There  are 
besides,  in  the  neighborhood  of  this  light,  small  cloudlets,  the  violet- 
red  color  of  which  is  exactly  the  same,  except  as  regards  intensity, 
as  that  of  the  anti-solar  light.  An  irregular  corona  around  the 
moon,  fringed  and  elongated  in  certain  directions  by  cirrus-streamers 
to  the  four  points. 

it 

14 

21^ 

A  very  fine  and  very  hot  day.     S.  and  A.  S.  very  fine. 

fi 

15 

27^ 

A  very  fine  and  very  hot  day.     S.  and  A.  S   feeble. 

it 

16 

35^ 

"        day  and  slight  cirrus.     No  S.  and  A.  S. 

(( 

■n 

21  ^ 

"        cirrus  and  cirro-cumulus.     At  night  storm. 

n 

18 

17^ 

Quite  fine  in  the  morning.  Cloudy  in  the  evening.  Rain  and  storm 
at  6  o'clock  P.M. 

ti 

19 

17^ 

A  day  divided  between  sun  and  clouds. 

ti 

20 

34^ 

A  fine  day.     S.  and  A.  S. 

a 

21 

29^ 

"                       "              Cirrus  and  cirro-cumulus. 

li 

22 

33^ 

Fine  day  without  clouds.     S.  and  A.  S.  fine  but  short-lived. 

23 

32^ 

Fine  day  without  clouds.     Fine  but  short-lived  S.  and  .A.  S. 

n 

24 

29^ 

Numerous  cirri  in  the  evening.  Cumulus  and  storm.  Barometer 
falls. 

a 

25 

1^ 

Dark  and  cold  day.     Lunar  halo  of  22°. 

n 

26 

13^ 

Middling  day.     Some  glimpses  of  the  sun. 

ti 

27 

oi 

Rainy  weather  in  the  morning.     Dark  and  cold  all  day. 

it 

28 

4^ 

A  few  glimpses  of  the  sun. 

u 

29 

3^ 

Fog  and  rain  all  day. 

it 

30 

2^ 

Rain  in  the  morning,  in  the  evening  fog.     No  sun. 

October 

I 

3^ 

Rain  in  the  morning  ;  very  little  sun  in  the  evening.  S.  and  A.  S. 
Violet  mist  in  the  valley. 

(1 

2 

12^ 

Quite  a  fine  day.  In  the  evening  violet  lights  very  perceptible  in  the 
neighborhood  of  Venus,  whose  brilliancy  is  very  great. 

t< 

3 

20^ 

Sky  cloudy  all  day. 

it 

4 

II  ^ 

Fine  in  the  morning  ;  in  the  evening  dark. 

tt 

5 

24^ 

A  fine  day.     Very  few  clouds.    S.  and  A.  S. 

tt 

6 

12^ 

Foggy  day. 

tt 

7 

1^ 

Rain  all  day. 

it 

8 

24^ 

Fine  day,  autumn  like.     Sun  rather  veiled. 

tt 

9 

13^ 

Rain  in  the  morning  ;  sun  in  the  evening. 

it 

10 

6^ 

Rain  all  day.     Rare  glimpses  of  the  sun. 

it 

II 

ni 

Rain  in  the  morning  ;  a  little  sun  in  the  evening. 

it 

12 

12^ 

Covered  sky  ;  rare  glimpses  of  the  sun. 

it 

13 

7^ 

Sky  overcast  ;  cold  weather.     North  wind.     Frost  at  night. 

It 

14 

22  ^ 

Fine  in  the  morning,  overcast  in  the  evening. 

it 

15 

Ai 

Dark  and  rainy  day. 

ti 

16 

13^ 

Rainy  in  the  morning  ;  in  the  evening  breaks  in  the  clouds. 

AND  THE  ACTINIC  CONSTITUTION   OF  THE  ATMOSPHERE. 


13 


Date. 

Combustion. 

Remarks. 

October 

>7 

19^ 

Very  fine  day,  from  beginning  to  end. 

i8 

t8^ 

"         "       "     In  the  evening  cirrus  and  halo  of  22°.     Barometer  falls. 

>9 

18^ 

Dark  in  the  morning,  a  little  sun  in  the  evening.  Incessant  rain  after 
3  o'clock  P.M. 

20 

5^ 

Rainy  day.     No  sun. 

21 

9^ 

ii                i< 

22 

4^ 

it                it 

23 

24^ 

Quite  a  fine  day,  in  spite  of  east  wind  which  rose  very  high  after 
9  o'clock. 

24 

3i 

Rainy  day  from  beginning  to  end. 

25 

14^ 

Partly  sun  and  partly  rain.  Two  currents  in  the  air,  one  from  the 
south,  superior,  carrying  off  cirri  ;  the  other  from  the  north,  inferior, 
with  clouds.  The  latter  finally  dominates  and  after  having  brought 
up  intermittent  and  slight  rains,  it  gives  a  cool  night. 

tt 

26 

2^ 

A  rainy  day. 

i( 

27 

-5^ 

Stormy  at  night.     Day  quite  fine. 

a 

28 

3J^ 

Rainy  day. 

a 

29 

5^ 

"          Rare  glimpses  of  light. 

These  two  months  of  iininteirupted  obsei-vation  prove  already  that  tlie  solar 
combustion  passes  through  very  different  values  within  24  hours.  These  changes 
are  sometimes  very  sudden  and  exceed  especially  those  of  the  thermometer,  the 
bai'ometer,  and  even  those  of  the  avei-age  brightness  of  the  day.  The  actinometric 
effect  does  not  show,  therefore,  that  approximate  constanc}%  which  makes  it  rela- 
tively so  easy  to  measure  the  other  effects  of  solar  I'adiation  ;  it  requires  a  veiy 
close  and  minute  investigation. 

While  it  amounts  to  little  or  nothing  at  all  in  overcast  and  rainy  weather,  it 
rises  very  perceptibly  during  fine,  sunny  days;  but  it  seems  to  be  subject  to  other 
influences  yet  beside  those  which  we  have  mentioned  when  we  spoke  of  "a  fine 
day,"  "  fine  weather,"  etc.  If  we  find,  in  fact,  that  the  days  from  tiie  20th  to  the 
24th  September  i-esemble  each  other  very  closely,  as  far  as  their  external  physi- 
ognomy is  concerned,  and  are  also  very  much  alike  in  point  of  actinoiuetry,  we 
have  on  the  other  hand  the  example  of  October  17th,  18th,  and  19th,  during 
which  the  degree  of  combustion  was  the  same,  and  this  although  the  weather  had 
been  very  fine  during  the  first  two  days  and  very  indifferent  during  the  last.  An 
instance  of  the  opposite  nature  is  offered  to  us  by  the  6th,  7th,  and  8th  of  Septem- 
ber, which  diffeied  veiy  much  in  their  actinometric  aspect,  whilst  they  lesembled 
each  other  so  far  as  their  external  physiognomy  was  concerned. 

It  would  be  interesting  to  find  out  under  what  influences  these  variations  are 
produced.  In  the  meantime,  until  we  reach  that  point,  let  us  notice  that  the  com- 
bustion on  the  finest  days  in  October  does  not  amount  to  as  much  as  that  obtained 
on  the  finest  days  in  September,  and  that  the  latter  again  do  not  equal  the  fine 


14  ATMOSPHERIC  ACTINOMETRY 

days  in  August,  mentioned  on  page  8,  if  we  bear  in  mind  that  the  expei'iments 
mentioned  on  that  page  were  made  in  a  cone-shaped  glass,  capable  of  holding  the 
bulb  of  a  thermometer  and  not,  like  those  made  in  September,  with  shallow  vessels 
in  which  the  figures  would  have  been  much  higher. 

One  might  be  tempted  to  see  here  the  effect  of  the  lessened  length  of  days. 
But,  in  order  to  avoid  this  influence,  the  length  of  exposure  has  been  everywhere 
precisely  the  same :  from  8.30  a.m.  to  4.30  p.m.  There  is,  therefore,  an  influence 
due  to  the  seasons,  which  we  must  also  endeavor  to  trace  back  to  its  true  cause. 
For  this  purpose  we  can  only  collect  the  gi-eatest  possible  amount  of  evidence. 

RESULTS    OF    OBSERVATIONS    MADE   IN    1886    AND    1887. 

I  made  for  this  end  several  series  of  experiments  in  1886  and  1887,  at  Paris, 
in  the  Cantal,  and  at  Orcines,  at  the  foot  of  the  Puy-de-Dome.  Ilnfoi'tunately  I 
cannot  report  them  here  in  detail,  having  mislaid  the  papers  which  contained  the 
record.  I  can  only  indicate  the  general  results  which  I  have  retained  in  my 
memory,  because  they  served  as  a  starting-point  for  new  investigations.  In  the 
first  place  I  again  found  evidence  of  the  almost  perfect  independence  between  the 
degree  of  solar  combustion  of  oxalic  acid,  and  the  occurrence  of  solar  and  anti-solar 
lights.  If  there  are  any  examples  of  coincidence  between  an  active  combustion 
and  the  presence  of  such  lights,  it  is  because  these  lights  appear  only  in  fine 
weather.  But  there  are  also  other  cases  in  which  combustion  is  very  rapid  and 
when  those  lights  are  altogether  missing.  If  they  play  any  part  at  all,  it  seems  to 
be  one  quite  secondary,  and  this  view  agrees  very  fairly  with  the  hypothesis  that 
the  phenomenon  is  due  to  the  presence  of  aqueous  vapors  in  the  upper  regions  of 
the  "atmosphere.  It  is,  of  course,  well  known  that  liquid  water  or  water  in  the 
form  of  steam  influences  the  activity  of  actinic  combustion  very  little. 

Another  very  important  result  is  this :  that  the  maxima  of  the  figures  of  com- 
bustion during  the  finest  days  are  higher  in  spring  than  in  summer.  The  difference 
did  not  stiike  me  as  quite  so  marked  as  between  summer  and  autumn.  As  to  the 
rriaximum  in  spring  I  have  always  found  it  very  clearly  marked  during  observa- 
tions carried  on  for  four  years,  and  an  example  of  this  will  be  found,  unfortunately 
too  limited  in  its  nature,  when  I  shall  speak  of  my  experiments  of  1888. 

The  maximum  in  spring  appears  alike  in  Paris  and  in  the  country.  But  I 
have  also  found  that  solar  combustion  was  less  intense  in  Paiis  than  in  the  Cantal 
or  in  the  Puy-de-Dome;  this  difference  appears  not  only  in  the  high  figures 
connected  with  the  oxalic  acid,  but  I  have  found  it  also  in  a  long  series  of 
experiments,  which  I  had  undertaken  in  order  to  study  the  transformations  which 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  15 

several  organic  substances  undergo  in  the  solar  light,  and  which  offer  a  greater 
resistance  than  oxalic  acid.  These  I  had  been  compelled  to  leave  in  the  light  for 
weeks  and  for  months  befoie  the  attack  was  complete.  They  thus  summed  up  the 
influence  to  which  they  had  been  subjected  during  the  length  of  the  exposure. 
Now  this  phenomenon  required,  generally,  for  its  termination,  much  more  time, 
sometimes  three  oi-  four  times  more,  in  Paris  than  in  the  country. 

Among  the  facts  of  this  kind  I  can  only  quote  one  which  I  find  I  had  by 
chance  inserted  in  a  work  intended  for  the  examination  of  another  question.  A 
deci-noi'mal  solution  of  tai'taric  acid  which  was  every  day  exposed  from  10  a.m.  till 
2  P.M.  to  the  sun  in  Paris,  had  lost  by  combustion  in  seven  months  and  a  half  only 
10  per  cent  of  its  acid,  while  in  the  Cantal  an  identical  solution  had  lost  in  two 
months  47  per  cent.  This  involves  a  combustion  about  fifteen  times  more  rapid, 
and  although  the  length  of  exposure  was  a  little,  greater  every  day  in  the  Cantal 
than  in  Paris,  and  although  the  quantum  of  solar  combustion  increases  moi-e  rapidly 
than  the  length  of  exposure,  this  is  not  enough  to  make  up  for  the  difference.  In 
another  case  involving  the  combustion  of  glucose  in  an  alkaline  liquid,  I  found 
that  to  take  two  yeai's  in  Paris  which  had  required  only  three  months  in  the 
Cantal. 

Finally,  this  experiment  teaches  us  also  that  average  years  do  not  resemble 
each  other  and  that,  if  thei-e  are  some  which  are  rich  in  chemical  radiations,  there 
are  poor  ones  also.  These  differences  between  one  year  and  another,  from 
this  point  of  view,  appeared  to  me  more  marked  than  in  any  other  respect.  We 
have  shown  above  the  most  striking  inequalities  between  consecutive  days  of  the 
same  season.     They  recur,  less  markedly,  for  consecutive  years. 

These  statements  which,  I  repeat,  I  regret  not  being  able  to  support  by 
figures,  suggest  a  number  of  problems  for  which  I  have  begun  to  seek  a  solution. 

In  the  first  place,  the  fact  that  active  combustion  is  stronger  on  fine  days  in 
spring  than  in  summer  and  in  autumn,  shows  that  there  must  be  another  cause  of 
action  than  the  influence  of  temperature,  or  the  height  of  the  sun  above  the  horizon. 
We  are  naturally  led  to  think  of  the  influence  of  the  volatile  oi'ganic  products 
which  vegetation  scatters  in  the  air  during  summer,  and  which,  if  they  are  capable 
of  being  oxidized,  absorb  and  utilize  for  their  own  benefit  the  chemical  radiations 
of  the  light  which  passes  through  the  atmosphere,  preventing  them  from  reaching 
the  soil.  We  are  confirmed  in  this  view  by  what  has  been  said  before  (page  6) 
concerning  the  relative  poverty  of  solar  light,  at  the  time  when  it  reaches  us,  in 
radiations  able  to  oxidize  oxalic  acid. 

In  the  second  place,  the  difference  between  the  sum  total  of  the  annual  radia- 
tions at  Paris  and  at  the  Cantal,  or  on  the  high  table-lands  of  the  Puy-de-Dorae, 


16  ATMOSPHERIC  ACTINOMETRY 

leads  us  to  ask  if  the  question  of  altitude  may  not  perhaps  be  of  importance.  Two 
identical  vessels,  containing  the  same  solution  and  exposed  during  the  same  time 
at  different  heights  in  the  atmosphere — will  they^or  will  they  not  undergo  the 
same  degree  of  oxidation  ? 

INFLUENCE   OF   ALTITUDE. 

I  begin  with  the  last  question,  because  the  documents  which  helped  me  to 
solve  it  were  lost,  together  with  those  which  gave  the  results  of  the  experiments 
already  mentioned,  and  I  must  therefore  be  very  brief  in  my  treatment. 

In  order  to  solve  this  pi'oblem  I  installed  myself  at  the  foot  of  the  Pny-de- 
Dome  in  the  little  village  of  Orcines,  and  I  made  a  number  of  expeiiments  simul- 
taneously in  the  garden  of  the  house  in  which  we  lived,  and  on  the  terrace  of  the 
observatory  on  the  Puy-de-Dorae,  where  M.  Humandon  kindly  undertook  to  expose 
and  to  remove  again  at  certain  fixed  houi-s  the  vessels  containing  the  oxalic  acid 
which  had  been  rendered  sensitive.  ,The  two  stations  are  distant  from  each  other 
4  kilometres  in  a  dii'ect  line.  The  vertical  difference  amounts  to  about  400  metres. 
The  incline,  therefore,  between  the  two  stations  does  not  count  foi-  much,  and  they 
cannot  be  considered  as  being  upon  the  same  vertical  line.  Experiments  made  on 
the  toj)  and  at  the  foot  of  the  Eiffel  tower  would  have  been  more  satisfactory  in 
this  respect.  But  at  the  Eift'el  tower  I  should  have  had  to  appi'ehend  encounter- 
ing difficulties  of  another  kind,  especially  the  want  of  homogeneousness  between 
the  layers  of  the  atmosphere.  For  the  lower  ones  which  had  swept  populous  parts 
of  the  city  could,  in  that  amount,  no  longer  be  considered  equivalent  to  the  upper 
parts.  What  tempted  me  to  choose  the  station  of  the  Puy-de-Dome  was  exactly 
this  homogeneousness  of  the  whole  region  so  far  as  its  vegetation  was  concei-ned. 
The  Puy-de-I)ome  is  suri-ounded  to  a  great  distance  by  a  dry,  almost  deserted 
countiy,  covered  with  woods  and  lai'gely  with  heathei-,  while  some  portions  are 
absolutely  bare  in  places  where  pozzolanes  crop  out  of  the  soil  or  in  those  immense 
overflows  of  lava  of  recent  date,  which  are  called  "  cheires,"  and  which  defy  any 
attempt  at  cultivation.  One  of  these  "cheires"  cropped  out  close  to  the  house  in 
which  I  dwelt,  and  I  imagine  that,  on  the  whole,  there  was  no  reason  why  the  air 
on  the  top  and  that  of  the  table-land  from  which  the  mountain  I'ises,  should  be 
heterogeneous.  In  spite  of  these  favorable  circumstances  I  did  not  find  that  the 
combustion  at  the  observatory  was  very  different  from  that  at  Orcines.  It  exceeded 
the  latter  a  little,  on  an  average,  but  with  exceptional  results  in  one  or  the  other 
direction,  so  as  to  prevent  any  positive  conclusion.  I  remember  that  my  estimates 
showed  an  increase  of  altitude  accompanying  an  increase  of  actinic  intensity,  but 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  17 

that  they  were  not  such  as  to  pi-ove  it.     This  conclusion  diminishes  my  regret  at 
having  mislaid  the  data. 

INFLUENCE    OF    OXIDIZABLE    SUBSTANCES    SUSPENDED   IN   THE    AIR. 

With  this  second  question  I  have  been  more  successful,  since  comparative  ex- 
periments can  here  be  made  with  far  more  precision  than  elsewhere.  All  that  is 
necessary  is  to  expose  in  one  and  the  same  place  two  vessels  containing  the  same 
oxalic  solution ;  one  being  made  to  float  upon  water  contained  in  a  deep  crystal- 
lizing pan  in  such  a  way  tliat  a  layer  of  stagnant  aqueous  vapor  may  be  kept 
above  the  surface  of  the  liquid  which  it  contains.  The  other  vessel  floats  in  like 
manner,  and  under  the  same  conditions,  on  the  surface  of  some  turpentine  or  of 
any  other  essential  oil.  It  is  always  found  that  combustion  is  far  less  advanced  in 
the  second  than  in  the  first  vessel.  As  I  said  before,  I  lost  the  relative  figures  of 
the  results  obtained  by  the  experiments  made  in  1886  and  1887.  But  Mr.  Elf- 
ving,  professor  at  the  University  of  Helsingfors  (Finland),  to  whom  I  had  men- 
tioned the  results  thus  obtained,  began  once  more  to  experiment  with  essence  of 
turpentine,  and  I  will  here  quote  the  results  as  he  reported  them  to  me  in  a  letter 
which  I  have  fortunately  preserved  : 

"I  have  repeated  and  confirmed  your  experiments  on  August  30,  1888,  from 
8  A.M.  to  4  P.M.  with  a  clear  sky.  There  wei-e  53  per  cent  of  the  oxalic  acid  burnt 
above  the  water,  and  39  per  cent  above  a  bath  of  essence  of  turpentine.  The  next 
day,  which  remained  clear  from  9  a.m.  till  noon,  the  figures  were  47  per  cent  and 
20  per  cent  for  the  same  length  of  exposure.  It  is  certain,  therefore,  that  the 
presence  of  oxidizable  substances  in  the  air  possibly  diminishes  the  consuming 
power  of  the  sun." 

Mr.  Elfving  has  confirmed  this  conclusion  by  the  following  experiment,  which 
I  have,  in  my  turn,  repeated  and  confirmed.  It  consists  in  sifting,  so  to  speak,  the 
rays  of  the  sun  through  a  solution  of  sul^ihate  of  quinine,  which  absorbs  a  part  of 
the  chemical  radiations  before  they  can  react  on  the  oxalic  solution.  Another  sift- 
ing apparatus,  of  the  same  thickness,  but  consisting  of  pure  water,  furnishes  a 
standard  of  comparison.  The  latter,  rigorously,  might  be  neglected,  for  the  quan- 
tity of  watery  vapor  or  of  liquid  or  solid  water  which  the  rays  of  the  sun  have 
traversed  before  reaching  the  level  of  the  soil,  exceeds  by  far  the  thickness  of  the 
supplementary  screen  of  liquid  ;  the  absorption  due  to  water,  is,  moreover,  very 
feeble.  In  my  experiments  I  suppressed  this  complication.  Mr.  Elfving  used  two 
glass  bell  jars  with  double  walls,  of  which  one  contained  water,  the  other  a  solu- 
tion of  sulphate  of  quinine.     He  wrote  to  me  on  June  17th : 


18  ATMOSPHERIC  ACTINOMETRY 

"  The  light  which  has  gone  through  a  layer  of  water  is  five  times  moi-e  active 
than  that  which  has  traversed  a  solution  of  sulphate  of  quinine  of  quite  the  same 
depth.     I  shall  continue  my  observations  at  the  time  of  the  solstice." 

And  on  July  9th  : 

"  I  have  again  made  an  experiment  with  sulphate  of  quinine.  On  June  27th, 
while  there  were  consumed  in  the  open  air  during  the  whole  day  87  per  cent  of 
the  total  amount  of  oxalic  acid,  and  78  per  cent  undei-  a  bell  jar  filled  with  water, 
the  decomposition  amounted  only  to  20  per  cent  under  a  precisely  similar  bell 
filled  with  a  solution  of  sulphate  of  quinine." 

Analogous  results  are  obtained  by  comparing  the  effect  of  sifting  solar  rays 
through  a  solution  of  potasssium  bichromate,  which  by  pi-efei'ence  allows  those 
radiations  to  pass  which  are  least  i-efrangible,  with  that  of  transmission  through  a 
solution  of  sulphate  of  copper  which  allows  the  most  refrangible  radiations  to  pass. 
All  this  proves  that  it  is  mainly  the  chemical  ladiations  which  are  of  importance, 
and  that  when  these  radiations  ai-e  employed  in  oxidation,  or  more  generally  in  the 
transformation  of  organic  or  even  mineral  substances  in  the  air,  they  reach  the  sur- 
face of  the  soil  much  weakened. 

Here  is,  therefore,  a  local  cause  of  variations  in  the  actinometric  degree ;  a 
local  and  incidental  cause,  considering  that  it  may  be  summed  up  thus  :  There  may 
exist  actinic  clouds,  clouds  scarcely  visible  to  the  naked  eye  and  not  accessible  to 
our  senses,  but  the  effect  of  which,  at  least  as  far  as  it  can  be  measured  by  solu- 
tions of  oxalic  acid,  exceeds  by  far,  in  intensity,  that  of  the  variations  in  brilliancy 
and  obscurity  produced  by  ordinaiy  clouds.  These  clouds  come  and  go,  are  no 
longer  to-day  where  they  wei'e  the  day  before;  they  dissolve,  for  they  are,  like 
other  clouds,  no  sooner  formed  than  they  become  subject  to  ceaseless  causes  of 
destruction.  This  explains  very  fully  why  the  actinometric  degree  should  vary 
so  greatly  from  day  to  day  and  from  one  year  to  another.  It  may  also  be  that  we 
find  here  our  explanation  of  the  greater  actinic  power  which  spring  has,  in  other 
words,  that  season  during  which  the  atmosphere  is  certainly  pooi'est  in  organic 
substances. 

Upon  reaching  this  point  we  see  new  vistas  open  before  us.  It  is  well  known 
that  the  turning  green  of  the  foliage  and  the  production  of  chlorophyll  may  take 
place  when  the  intensity  of  light  is  very  feeble,  as  for  instance  in  the  back  of 
a  room  lighted  by  one  window,  but  that,  under  such  circumstances,  the  chlorophyll 
does  not  begin  to  act  and  is  not  decomposed  by  carbonic  acid.  It  requires  a  much 
stronger  luminous  intensity  for  the  process  of  assimilation  to  begin.     This  phe- 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  19 

noraenoa  increases  with  intensity  of  light  up  to  a  certain  raaximxim,  differing  in 
different  plants ;  beyond  this  it  decreases.  Starting  from  this  point,  it  is  naturally 
suggested  that  those  actinic  clouds  which  we  have  just  discovered,  cannot  be  with- 
out influence  on  vegetation,  since  they  modify  so  largely,  although  often  invisibly, 
the  strength  of  the  chemical  i-adiations,  which  is  nearly,  if  not  absolutely  the  only 
active  element  in  the  complex  whole,  which  until  now  has  been  studied  under  the 
name  of  Luminous  Intensity.  And  if  again  the  plants  themselves  disperse  into 
the  air  the  materials  which  absoib  the  solar,  chemical  radiations,  how  can  we 
avoid  thinking  that  possibly  the  production  of  these  odoi'iferous  and  oxidizable 
effluvia  may  be  for  the  plant  a  means  of  protection  ? 

To  elucidate  this  subject  fully,  would  require  expei'iments  which  I  have  not 
the  time  to  make.'  I  have  been  satisfied  with  examining  it  under  various  aspects. 
Odorous  and  essential  oils  are  not  alone  aide  to  ai'i'est  the  passage  of  chemical  radi- 
ations. The  surfaces  of  plants  aie,  as  is  well  known,  frequently  covered  with  a  fatty 
or  waxy  layer,  which  is  highly  oxidizable.  There  are,  besides,  at  all  times  fatty 
substances  in  the  air,  as  is  pioved  by  the  greasy  feel  of  old  dust  deposited  upon 
our  furnitui'e.  What  effect  can  these  fatty  substances  exert  upon  the  combustion 
of  oxalic  acid  in  the  sunlight  ?  If  our  ideas  are  correct,  a  slight  layer  of  fatty 
matter  should  protect  that  acid  against  solar  light. 

INFLUENCE   OF    FATTY    SUBSTANCES. 

The  presence  of  fatty  matter  on  the  surface  of  our  test  solution  brings  up  a 
slight  experimental  difficulty.  It  is  this,  that  solar  oxidation  of  a  fatty  substance 
is  always  accompanied  by  a  production  of  acid  which  raises  the  titre  of  the  oxalic 
solution  at  the  same  time  that  the  solai-  combustion  lessens  it.  We  must,  therefore, 
either  use  a  very  small  quantity  of  fatty  matter  so  that  it  may  barely  form  an  im- 
perceptible veil  to  cover  the  liquid,  or,  what  is  better  still,  we  must  spread  it  out 
in  a  transparent  layer  over  a  surface  of  glass  interposed  in  the  path  of  the  luminous 
rays.     Here  are  some  experiments  made  in  connection  with  this  subject: 

Map. — On  June  27,  1885,  I  exposed  to  the  sun  during  six  hours  seven  vessels 
of  the  same  dimensions,  containing  each  10  c.  c.  of  oxalic  acid  in  half-deci- 
normal  solution.  Two  of  these  vessels,  Nos.  1  and  2,  had  their  walls 
clean.  Vessel  No.  3  had  been  rubbed  with  a  weak  solution  of  butter  in 
sulphide  of  carbon,  which  left  upon  the  sides,  hardly  tarnishing  them,  a 
greasy  layer.  Moreover,  by  virtue  of  a  well-known  phenomenon  of  super- 
ficial tension,  the  walls  of  the  vessel  have  allowed  an  invisible  layer  of 
fatty  matter  to  spread  on  the  surface  of  the  liquid.     In  order  to  separate 


20  ATMOSPHERIC  ACTINOMETRY 

this  action  as  far  as  possible  fi-om  that  of  the  roughness  of  the  walls,  a 
fourth  vessel  is  brought  up  to  the  same  degree  of  opacity  as  No.  3,  by 
rubbing  it  externally  with  chalk  diffused  in  water.  Finally,  to  increase 
the  quantity  of  fatty  matter  pi-esent  in  the  liquid,  and  also,  in  order  to 
see  the  effect  which  a  little  oj^acity  of  the  liquid  may  there  produce,  new 
vessels,  Nos.  1',  2',  and  3',  have  been  prepared  like  the  vessels  Nos.  1,  2, 
and  3,  simply  adding  to  each  two  drops,  in  other  woids  5  milligrammes, 
of  fatty  matter. 

These  were  the  results  : 

Combustion. 

Vessel  No.  i,  clean  sides 33  ^ 

"     2,      "        "     33^ 

"         "     3,  dull  sides,  greasy  surface 29  ^ 

"     4,     "       "       (chalk) 32^ 

"         "     i",  like  I,  plus  2  drops  of  milk 16  ^ 

"   2',  "  2,  "   "    "         "  17  $?; 

"         "     3'.    "     3,    "     "      "  '•    17^ 

Other  experiments,  made  the  following  yeai-s,  and  the  detailed  repoi-ts  of 
which  have  been  lost,  confirm  these  first  results,  and  show  that  the  fatty  matter 
contained  in  a  liquid  or  spread  as  an  invisible  layer  over  the  surface,  like  that 
which  covers  the  walls  of  a  bell  jar  placed  over  the  vessel  containing  oxalic  acid, 
diminishes  the  actinic  effect  of  the  solar  rays. 

Finally,  it  is  the  same  with  many  substances,  more  or  less  easily  oxidizable, 
which  also  exert  a  protecting,  or  at  least  a  retarding,  effect  upon  the  influence  of 
the  chemical  radiations.     Such  is,  for  instance,  alcohol. 

Mcp. — On  the  26th  June,  1885,  two  vessels  with  10  c.  c.  of  an  oxalic  acid  solu- 
tion, containing  J^  equivalent  per  litre,  gave  me  a  combustion  of  37  per 
cent,  the  same  for  both.     Two  other  vessels,  exactly  alike,  which  have 
received  an  addition  of  2.5  c.  c.  alcohol  of  90  per  cent,  gave  me  only  a 
combustion  of  21  per  cent. 
Mop. — On  September  14,  1888,  two  vessels  with  a  solution  of  ^^  equivalent  of 
oxalic  acid,  gave  me  identical  combustions,  rising  as  high  as  10  per  cent. 
They  amounted  only  to  4  per  cent  in  two  like  vessels,  to  which  a  few  drops 
of  oil  of  oranges  had  been  added,  so  that  the  essential  oil  and  the  alcohol 
have  acted  similarly. 
I  have  made  numerous  experiments  with  divers  substances  which  were  oxidi- 
zing or  oxidizable,  the  details  of  which  have  been  lost.     In  a  general  way  I  have 
found  that  the  former  increase  the  combustion  of  oxalic  acid,  while  the  others 
retard  it. 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  21 

But  tliis  rule  is  not  always  confirmed,  on  account  of  the  intervention  of  what  I 
have  called  in  another  paper  phenomena  of  entanglement.  These  operate  so  that 
one  oxidizable  body  may  involve  another  in  the  same  decomposition  which  it 
undergoes  itself.  But  here  we  begin  to  stray  away  from  the  subject  of  this  paper, 
which  is  the  actinic  study  of  the  atmosphere.  I  am  content,  therefore,  to  draw 
from  the  great  sum  total  of  the  i-esults  I  have  obtained  the  following  conclusions: 
The  nature  and  the  proportion  of  the  oxidizable  elements  which  living  nature  scat- 
ters through  the  air,  })etray  themselves  in  the  solar  combustion  of  oxalic  acid,  which 
is  the  more  feeble  on  the  surface  of  the  solution  the  more  the  radiations  have  met 
with  unstable  elements  to  oxidize  dui'ing  their  passage.  The  organic  substances  of 
the  atmosphere  are  therefore  a  protection  against  too  intense  an  action  of  the  chemi- 
cal rays  at  the  surface  of  the  soil,  and  the  effect  which  they  produce  is  not  only 
measurable,  but  sometimes  very  great.  In  other  woi'ds,  we  do  not  know  what  the 
chemical  power  of  solar  light  may  be  at  its  entrance  into  the  atmosphere,  but  on  a 
level  with  the  soil  it  is  so  impoverished  that  a  thin  layer  of  turpentine  vapor,  of 
sulphate  of  quinine,  or  of  any  oxidizable  substance,  suffices  to  destroy  it  almost 
completely. 

This  conclusion  has,  however,  another  side  to  be  considered,  which  is,  that 
the  atmosphere  must  at  every  moment  be  the  seat  of  combustions,  such  that,  on 
the  whole,  all  luminous  radiations  are  utilized.  I  shall  not  insist  here  on  the  power 
and  the  importance  of  the  phenomena  of  oxidation  which  take  place  in  the  atmos- 
phere and  at  the  level  of  the  soil,  nor  upon  the  genei'al  effect  which  they  have  on 
sanitation  over  which  they  pi'eside.  1  have  published  several  memoirs  on  that  sub- 
ject,' to  which  I  must  be  content  to  refer.  I  have  there  called  attention  to  the 
power  of  the  solar  rays  on  microbes,  first  weakening  and  then  killing  them,  a 
power  which  was  first  indicated,  but  incompletely  proven,  by  Messrs.  Downes  and 
Blunt.'  I  have,  moreover,  studied  the  influence  of  the  conditions  of  the  medium 
on  the  resistance  of  germs.  All  that  has  been  done  since,  has  only  confirmed  the 
importance  which  I  attached  to  light  and  to  the  chemical  portion  of  the  solar  spec- 
trum as  principal  agents  in  the  hygiene  of  the  globe. 

INFLUENCE    OF   INCREASE    OF    LATITUDE. 

This  first  problem,  that  of  the  possible  influence  exerted  by  oxidizable  sub- 
stances while  in  suspension  in  the  atmosphere,  having  been  sufficiently  discussed  in 

'  Annales  de  Chimie  et  de  Physique,  6th  S.,  vol.  v.,  May,  1885,  and  Comptes  Rendus,  vol.  c.  and  ci. 
Annales  de  V Institut  Pasteur,  vol.  I.,  p.  88. 

'  The  conclusions  of  these  scholars  had  been  opposed  by  Tyndall  and  by  Jamieson,  so  that 
when  I  took  up  the  question  anew,  it  had  not  yet  met  with  a  solution.     It  has  found  one  to-day. 


22  ATMOSPHERIC  ACTINOMETRY 

the  statements  just  made,  I  found  myself  face  to  face,  as  Professor  of  Meteorology, 
with  the  following  question  : 

It  is  an  acknowledged  fact,  that  the  activity  of  the  vegetative  process  in  the 
northern  parts  of  Eui'ope  is  very  great.  The  interval  between  sowing  and  harvest- 
ing, for  spring  wheat,  lasts  on  an  average  145  days  in  Alsace.  According  to  M. 
Tisserand  it  amounts  to  only  133  days  at  Halsno,  in  59°  30'  N.  latitude  ;  and  it  is 
only  114  days  at  Skibbotten,  in  69°  30'  N.  latitude.  It  decreases  therefore  as  we 
approach  the  pole  ;  notwithstanding  that  the  average  temperature  of  the  period  of 
vegetation  diminishes  likewise  with  the  increase  of  latitude. 

This  decrease  in  the  number  of  days  needed  for  vegetation,  as  we  draw  nearer 
the  north  pole,  seems  to  be  a  general  law.  Accoi'ding  to  Arnell,  barley  requires 
117  days  to  grow  in  Southern  Sweden,  92  in  Middle  Sweden,  and  89  in  Lapland. 
It  is  true  that  these  variations  depend  in  part  at  least  on  the  power  which  the  plant 
has  to  adapt  itself  to  external  conditions,  for  if  sown  in  our  country  the  Norwegian 
grain  grows  more  rapidly  than  ours,  while  our  own  native  grain,  carried  to  Norway, 
lags  behind  the  acclimated  variety.  But  this  is  not  sufficient  to  explain  all,  and  we 
must  in  the  end  always  return,  as  a  final  analysis,  to  the  influence  of  climate. 

We  may  go  even  a  little  farther  in  our  induction.  According  to  Griesbach 
the  increased  rapidity  in  the  development  of  plants  cultivated  at  the  extreme  north 
does  not  affect  the  whole  evolution  of  the  plant,  but  only  the  period  between  ger- 
mination and  blooming.  It  applies,  therefore,  only  to  the  green  oi'gans  of  the  plant, 
and  thus  stai'ts  once  more  the  question  of  light,  which  actually  appears  to  be  of 
greater  importance  than  that  of  temperature.  In  fine,  to  return  to  the  subject  of 
our  Memoir,  the  actinic  influence  of  the  solar  rays  seems  to  increase  with  the 
latitude. 

To  what  is  this  increase  due?  This  question  is  still  open  and  to  it  I  have 
tried  to  find  an  answer.  The  first  point  to  determine  was  whethei'  the  solution  of 
oxalic  acid  also  showed  such  an  increase  of  actinic  effect  ? 

It  was  on  this  account  that  I  asked  Mr.  Elfving  to  assist  me,  whose  interesting 
experiment  I  have  mentioned  above  (page  17).  I  sent  to  him  at  Helsingfors  an 
oxalic  solution,  and  ten  vessels  exactly  alike,  such  as  I  had  used  myself  in  France, 
in  order  to  make  sure  that  at  least,  and  as  far  as  possible,  those  experimental  condi- 
tions which  we  could  control  should  be  as  identical  as  they  could  be  made. 

Unfortunately  there  were  other  conditions  which  wei'e  entirely  beyond  our 
control.  The  ideal  would  have  been  attained  with  a  series  of  days  equally  fine, 
occurring  simultaneously  in  France  and  in  Finland,  and  permitting  us  to  make  our 
observations  under  precisely  the  same  circumstances.  But  there  are  obvious  reasons 
why  the  weather  could  hardly  ever  be  the  same  in  France  and  in  the  Gulf  of 


AND  THE  ACTINIC  CONSTITUTION  OF  THE   ATMOSPHERE.  23 

Bothnia.  When  that  vast  mass  of  air,  in  I'elative  repose,  which  I  have  named  The 
Isle  of  Calms,  rests  over  our  part  of  the  world  and  gives  us  fine  weather,  the  equa- 
torial current,  which  turns  it  northward,  is  over  Sweden  and  Norway,  to  which  it 
brings  overcast  skies  and  I'ains ;  when,  on  the  other  hand,  the  Isle  of  Calms  rests 
over  the  north  of  Europe,  we  are  in  France  subject  to  stormy  disturbances  which 
come  to  us  through  the  Mediteri-anean  or  the  Gulf  of  Gascony,  or  we  are  subject  again 
to  the  return  current,  which,  after  having  rounded  the  "  Island,"  comes  back  to  us  in 
the  shape  of  cold  east  and  northeast  winds.  To  find  favorable  coincidences  in  this 
grand  atmospheric  dance,  we  should  need  months  of  continued  observations,  which 
neither  Mr.  Elfving  nor  myself  were  in  a  condition  to  undertake. 

In  this  difiiculty  we  availed  oui'selves  of  the  meaning  of  the  word  "  fine  day," 
as  I  have  shown  above  (page  13),  which  is  so  uncertain  as  to  its  actinometric  defini- 
tion that  every  effort  to  make  it  absolute  as  to  perfect  equality  of  expei'imental  con- 
ditions becomes  rather  illusory.  We  could  be  content,  more  modestly,  with  a  first 
approximation  ;  it  was  enough  to  compare  the  actinometric  combustion  of  the  finest 
days  in  the  Gulf  of  Finland  and  in  France,  at  the  same  time  of  the  year. 

Nor  is  this  all.  The  length  of  the  day  is  greater  at  the  North  than  at  the 
South  during  the  period  of  vegetation,  and  the  length  of  the  insolation  has,  we  all 
know,  a  direct  influence  on  the  relative  quantum  of  combustion.  Hence  I  requested 
Mr.  Elfving  to  make  every  day  two  sets  of  experiments,  one  with  vessels  exposed 
to  the  sun  from  8  a.m.  to  4  p.m.,  like  those  which  I  was  using  in  France,  and  the 
other  with  vessels  left  out  from  8  a.m.  till  the  setting  of  the  sun. 

Mr.  Elfving  made  at  Helsingfors  between  August  27th  and  September  4th,  1887, 
five  seiies  of  experiments,  which  I  cannot  compare  with  those  which  I  was  making  at 
the  same  time  at  the  foot  of  the  Puy-de-Dome,  and  the  records  of  which  have  been 
lost.  But  I  am  fortunately  able  to  compare  them  with  those  which  I  had  begged 
M.  Ch.  Mascart  to  make  at  the  same  time  near  the  seashore  in  the  Channel.  These 
may  perhaps  be  better  fitted  for  comparison  with  those  made  by  Mr.  Elfving,  as 
both  were  made  at  maritime  stations.  All  that  I  noted  when  I  received  them  was 
that  they  gave  much  higher  figures  than  those  which  I  obtained  at  the  same  time 
on  the  bare  table-land  which  cariies  the  Puy-de-Dome. 

In  the  first  place,  here  is  a  table  of  the  observations  made  in  France ;  it  is 
formed  in  the  same  way  as  those  which  have  already  been  given  in  this  Memoir. 


24 


ATJIOSPHERIC  ACTINOMETRY 

ST.    PIERRE    LE    PORT,     1 887. 


Date. 

Combustion. 

Remarks. 

August  1  S 

44  ^ 

Clear  weather  till  lo  o'clock  p.m.  ;  later  cloudy. 

i6 

38^ 

Rain  till  2  o'clock  ;  later  overcast. 

'7 

31  i 

Very  clear  from  11  till  3  ;  afterwards  cloudy. 

18 

23  ^ 

Three-fourths  cloudy  till  10  a.m.     Clear  from  10  till  3  p.m. 

'9 

33  ^ 

Half  overcast  in  the  morning  -  then  quite  clear. 

20 

21  ^ 

Half  overcast  all  dav  long  ;  a  slight  fog. 

21 

28  <^ 

Slightly  overcast  in  the  morning  ;  then  clear. 

22 

3°  i 

Slightly  overcast  in  the  morning  ;  then  clear. 

23 

36^ 

Fine  weather. 

24 

29  ^ 

tt               a 

25 

42  i 

Warm.     Very  close.     Clear  weather. 

26 

32  ^ 

Covered  in  the  morning  and  evening.     Clear  from  12  till  3  p.m. 

27 

23  ^ 

Overcast.     Rain  from  10  till  11  a.m. 

28 

24^ 

Unceasing  rain. 

Here  asain  the  solar  coinbustiou  increases  with  the  fine  weather  and  diminishes 
when  the  sky  is  overcast  or  rain  falls.  Although  the  weather  was  on  an  average 
less  fine  than  during  the  corresponding  series  of  exi)erinients  cited  before,  the  latter 
gives,  on  the  average,  higher  results,  a  fact  which  confirms  what  we  have  already 
said  concerning  the  actinometric  differences  of  different  years  at  the  same  epoch. 
Here  are  now  the  experiments  made  by  Mr.  Elfving  at  the  same  time  in  1887 : 
Helsingfors,  latitude  60°  10'.  Length  of  day,  14  hours.  Height  of  the  suu 
above  the  horizon  at  noon,  about  38°.  ___ 


Solar  Combustion. 

Date. 

From  8  A.M.  to  4  P.M. 

All  day  long. 

August  27 
28 
29 

September  2 
4 

42^ 

53^ 
74!^ 
77^ 

55^ 

87^ 
89^ 

"  The  difference  between  the  first  three  days  and  the  two  others  is  quite  great ; 
it  arises,  no  doubt  from  the  fact  that  the  atmosphere  had  been  purified  by  heavy 
rains  on  August  30th  and  September  1st  and  3d.  In  March,  I  had  already  observed 
this  effect  of  rain."  (M.  Elfving.) 

The  figures  in  the  first  column  are  on  an  average  higher  than  those  which  cor- 
respond to  them  in  the  preceding  report,  and  this  superiority  must  be  all  the  more 


AND  THE  ACTINIC  CONSTITUTION  OF  THE   ATMOSPHERE. 


25 


striking,  as  by  a  mistake  in  our  agreement  the  exposure  to  the  sun  lasted  an  hour 
longer  in  France  than  in  Finland.  The  latter  ought,  therefore,  to  be  somewhat 
increased,  in  order  to  make  the  comparison  raoi-e  just.  We  shall  presentl)^  return 
to  the  results  marked  in  the  last  column.  Mr.  Elf\  ing  had  woi-ked  only  during 
five  days ;  the  comparative  experiments  were  therefore  not  immerous  enough,  and 
by  a  common  understanding  another  beginning  was  made  in  1888. 

Here  is  the  report  of  the  experiments  which  I  made  in  France,  in  the  garden 
of  the  Agronomic  Institute,  during  the  months  of  May  and  June,  1888.  My  official 
duties  prevented  me  from  making  tl\eni  in  an  unbroken  series,  and,  moreover,  I  had 
to  leave  out  three  observations  during  which  a  fierce  wind  suddenly  sprang  up  and 
covered  my  vessels  with  a  layer  of  dust. 


PARIS.       EXPOSURE  FROM  8  A.M.  '1  O  5   P.M. 


Date. 

Combustion. 

Remarks. 

May 

[2 

46^ 

A  fine  day.     Fresh  north  wind. 

« 

13 

29  ^ 

Very  fine  day  ;  rather  warmer  than  the  day  before. 

(< 

14 

5°!^ 

Cirrus  in  the  morning.     Very  fine  day. 

(( 

'5 

23^ 

Sky  overcast.     Barometer  falls. 

ts 

17 

52;^ 

"                   South  wind.     Cirrus. 

ft 

18 

23;^ 

tt                                          tt                           a 

n 

20 

27^ 

Quite  a  fine  day.     N.  wind.     Cirrus  and  alto-cumulus. 

(1 

21 

35!^ 

Sky  overcast.     Lighter  in  the  evening.     Fresh  east  winds. 

<( 

26 

43!^ 

g.  g.  clouds  ;  fine  at  night.     Fresh  north  wind. 

•t 

27 

30^ 

Quite  a  fine  day.     g.  g.  Cirrus.     Sudden  storm. 

June 

I 

33^ 

Fine  day.     No  wind. 

It 

2 

55;^ 

Warm  and  stormy  day. 

3 

39^ 

Very  warm  day.     South  wind. 

5- 

42^ 

Warm  and  stormy  day.     Sky  overcast. 

12 

64^ 

Fine  day.     A  little  air. 

The  correspondence  between  the  degree  of  combustion  and  the  state  of  the 
atmosphere  is  less  striking  in  these  observations,  which  were  made  in  Paris,  than  in 
those  made  in  the  country,  which  is  less  surprising  when  we  bear  in  mind  the 
incessant  heterogeneity  and  variability  of  the  air  in  a  large  city.  The  influence  of 
the  spring  season,  however,  to  which  reference  was  made  before  (page  14)  is  shown 
in  the  relative  magnitude  of  the  figures  of  combustion.  The  figure  64  %,  dated  on 
June  12th,  is  very  exceptional. 

Here  follow  next  the  results  obtained  almost  simultaneously  by  Mr.  Elfving 
at  Helsingfors : 


26 


ATMOSPHERIC  ACTINOMETRY 
HELSINGFORS.      EXPERIMENTS    MADE    IN     1 888. 


Solar  Combustion. 

Date. 

From  8  A.M.  to  4  P.M. 

All  (lay  long. 

May 

•9 

50^ 

-^ 

21 

47  ^ 

5M 

it 

22 

56^ 

76^ 

X     " 

a.l 

53^ 

65^ 

X     " 

24 

37  ^ 

55  ^ 

X     " 

27 

44^ 

• 

-^ 

ik 

30 

46^ 

72^ 

It 

31 

si;^ 

72  ^ 

Tune 

4 

48^ 

63^ 

X     " 

7 

48^ 

70^ 

tt 

8 

-^ 

74^ 

(1 

9 

56^ 

79  ^ 

to 

57^ 

77^ 

(( 

II 

54^ 

80^ 

On  the  days  marked  with  a  cross  (x),  the  sky  was  more  or  less  overcast  at 
Helsingfors.  All  the  figures  iu  the  second  column  ought  to  be  raised  slightly,  in 
order  to  make  them  fit  to  be  compared  with  those  in  the  preceding  table,  which 
correspond  to  an  additional  hour  of  exposure.  It  will  be  seen,  however,  that  they 
are  on  an  average  higher,  although  none  of  them  reach  the  exceptional  figure  of  the 
12th  of  June  at  Paris. 

The  conclusion  is  the  same  as  that  derived  from  the  experiments  of  1887.  In 
order  to  add  to  its  weight  we  recommenced  another  series  in  August  and  Septem- 
ber. This  time  I  installed  myself  on  the  Mont  Dore,  at  a  height  of  about  1100 
metres,  in  a  house  some  distance  from  that  little  village. 


MONT   DORE,    1 888. 


Date. 

Combustion. 

Remarks. 

August 

9 

26^ 

Cirrus  in  the  morning,  which  increased  towards  evening. 

lO 

19^ 

Fine  day. 

II 

18^ 

"          Sky  slightly  covered. 

12 

19^ 

13 

18^ 

Cumulo-cirrus  in  the  morning.     Fine  afternoon. 

14 

27^ 

Fine  day.     Sky  a  little  cloudy  in  the  evening.      South  wind. 

16 

^li 

Very  fine  day.     Atmosphere  limpid.     Cirrus  in  the  evening. 

17 

— 

Rain  all  day.     No  exposure. 

18 

— 

Sky  overcast,  and  rain. 

19 

22  <ji, 

Fine  day.     Cumulo-cirrus  and  cirrus. 

20 

15^ 

Cirrus  all  day,  especially  in  the  evening.     Barometer  not  falling. 

31 

15^ 

Large  white  cumuli. 

AND  THE  ACTINIC   CONSTITUTION   OF  THE  ATMOSPHERE. 


27 


Date. 

Combustion. 

Remarks. 

September   i 

15^ 

Same  weather  as  day  before. 

« 

2 

10  ^ 

Middling  day. 

(( 

3 

24^ 

Quite  a  fine  day.     Many  cirri. 

it 

4 

12^ 

Middling  day. 

« 

s- 

11^ 

tt 

(1 

6 

12  ^ 

Quite  a  fine  day.     Some  cumuli  early. 

*( 

7 

11^ 

Middling  day. 

(( 

lO 

11^ 

Quite  a  fine  dav  with  a  few  clouds. 

tt 

II 

7^ 

If             ft'            If             If 

tt 

12 

18^ 

Fine  day,  hot  sun,  a  few  cumuli. 

11 

'3 

25?^ 

Fine  in  the  morning,  middling  later. 

11 

14 

>S^ 

Fine  day,  very  warm. 

11 

IS 

9^ 

Very  fine  day,  as  yesterday. 

11 

i6 

4^ 

Middling  day.     Warm  and  heavy. 

II 

17 

29^ 

Day  divided  between  sun  and  clouds. 

It 

18 

Dark  day. 

tt 

19 

30^ 

Superb  day. 

'ti 

20 

10  ^ 

Sky  fine  early  ;  covered  in  the  evening. 

11 

21 

17^ 

Day  rather  finer  than  day  before. 

It 

22 

13^ 

Quite  fine  in  the  morning.     Cloudy  at  night. 

11 

26 

'S^ 

Cumulus  concealing  about  \  of  sky. 

11 

27 

25  ^ 

Rather  better  than  the  day  before. 

It 

28 

49^ 

Rather  a  dull  day,  but  no  clouds. 

Between  the  20th  and  30th  August  there  followed  a  long  period  of  rain  and  overcast  sky. 

What  strikes  us  in  reading  these  figures  is  their  smalluess  even  on  fine  days. 
They  are  the  smallest  I  have  ever  had  to  record  in  August  and  September,  on  an 
average,  and  this  although  the  latter  month  was  rather  fine  at  Mont  Dore  during 
1888  ;  there  is  also  to  be  noticed  a  great  lack  of  agreement  between  the  apparent 
character  of  the  day  and  its  actinometric  chai'acter.  Thus  the  very  fine  day  of  Sep- 
tember 15th  gave  only  a  combustion  of  9  per  cent,  when  the  slightly  veiled  day  of 
September  28th  gave  a  combustion  of  49  per  cent.  This  is  a  new  confirmation  of 
what  has  been  stated  before. 

I  partly  attribute  the  very  great  want  of  agreement  noticed  at  Mont  Doi'e  to 
the  fact  that  this  station  is  surrounded  on  all  sides  by  pine  woods  which  diffuse 
through  the  air  a  large  quantity  of  terebinthine  exhalations,  so  that  the  odoi-  be- 
comes sti'iking.  This  explanation  also  agrees  with  the  notions  which  I  have 
suggested  before.  Nevertheless  I  admit  that  it  would  require  very  special  com- 
parative experiments  to  establish  it  firmly,  and  to  draw  fi'om  it  the  proper  signifi- 
cation. We  must  be  content,  for  the  present,  to  remark  that  if  our  explanation  is 
correct,  it  will  also  account,  as  a  whole,  for  the  want  of  agreement  already 
mentioned.  If  the  exhalations  of  essential  oils  are  really  able  to  arrest  actinic 
radiations,  the  effect  of  what  we  call  a  fine  day  will  be  very  variable  according  as 
it  will  succeed  a  period  of  rains  which  may  have  washed  the  atmosphere,  as  in  the 


28 


ATMOSPHERIC  ACTINOMETRY 


observations  made  by  Mr.  Elfving,  or  as  it  may  come  to  us  after  a  warm  day  or  a 
period  of  great  heat,  which  may  have  increased  the  invisible  chmd  of  teiebinthine 
vapors  or  other  odorous  essences.  But,  I  I'epeat,  all  these  points  must  be  investi- 
gated directly,  and  this  preliminary  study,  although  it  has  continued  for  many 
years,  has  no  other  claim  than  that  of  suggesting  new  subjects  for  the  study  of  the 
atmosphere. 

Let  us  now  I'eturn  to  the  comparison  of  the  effects  which  equal  periods  of 
exposure  have  in  Fiance  and  in  Finland.  The  following  are  the  results  obtained 
by  Ml'.  Elfving  at  Helsingfoi-s,  duiing  the  same  period  of  the  same  year: 


HELSINGFORS,    l! 


Solar  Com 

justion. 

Viatt^ 

rv   Ol^l  1  T'lj  u 

l^dlc. 

From  8  A.M.  to4P..\f. 

All  day  long. 

ixt.  Ill (tr Ks, 

August        22 

56^ 

66^ 

Clear  sky. 

■'            ■23 

51  ^ 

60;^ 

Almost  clear 

26 

35^ 

45^ 

Cloudy. 

"               27 

56^ 

75^ 

Clear. 

28 

50  ^ 

68^ 

Half-overcast. 

•      "          ^9 

ssi 

71^ 

Very  clear  sky  from  9  a.m. 

"          30 

ii?o 

70^ 

Very  clear  sky  from  9  a.m.  to  12. 

"          31 

39!^ 

— 

a             tt              it              (t              (( 

September   2 

49^ 

59;^ 

Very  clear  sky. 

3 

49  ^ 

67^ 

(.         i( 

6 

54%; 

— 

Almost  clear. 

8 

49^ 

— 

Clear  early,  overcast  afterwards. 

9 

52  i 

62^ 

Very  clear  sky. 

10 

56^ 

-^ 

«         « 

II 

59^ 

— 

tt         it 

14 

51^ 

— 

Clear  sky. 

"         IS 

46^ 

— 

ii 

16 

51  ^ 

— 

u 

"         17 

5'^ 

— 

tt 

18 

42^ 

Clouds. 

The  regulai-ity  is  hei-e  greater  than  in  France,  and  what  is  especially  I'cmark- 
able  is  the  close  resemblance  in  an  actiiiometric  sense  of  the  days  which  are  marked 
as  "similar,"  in  the  column  of  "  Reiuarks  "  (Aug.  29th  and  30th,  Sept.  2d  and  3d, 
14th,  15th,  16th,  and  17th).  But  what  is  perhaps  most  striking  in  this  table,  when 
compared  with  that  on  page  26  is  that  the  figures  of  solar  combustion  are  notably 
higher  than  they  were  in  France  at  Mont  Dore,  at  the  same  time  of  the  year. 
Still,  Finland  is  very  rich  in  resinous  woods,  and  e\'en  if  the  station  were  less  sur- 
rounded by  them  than  at  Mont  Dore,  the  altitude  is  lower,  which  to  some  extent 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  29 

makes  partial  compensation.  Besides,  the  higher  figures  obtained  at  Helsingfors, 
persist  at  the  three  positions  of  the  Frencli  station  :  at  Paris,  on  the  coast  of  the 
Ciiannel,  and  on  the  mountain  of  Puy-de-Dome. 

There  can,  therefore,  remain  no  possible  doubt  on  this  point :  the  actinic 
intensity  of  light  in  noithern  countries,  close  to  the  soil,  is  greater  than  in  our 
temperate  zone  at  the  same  hours  of  the  day.  It  would  no  doubt  be  found  still 
weaker  if  we  approached  more  nearly  to  the  equator.  This  conclusion  was  alto- 
gether unforeseen. 

The  fact  once  established,  explanation  becomes  necessary.  This  greater  ac- 
tivity of  combustion  which  the  air  has  in  nortliern  regions,  might  be  ascribed  to 
ozone,  rendered  more  abundant  there  by  the  discharges  which  constitute  the 
aurora  boi-ealis,  and  more  active  by  the  action  of  light.  I  have  begun  to  study 
this  subject,  but  my  experiments,  interrupted  by  winter  and  my  retui'n  to  Paris, 
are  not  yet  completed,  and  I  shall  not  be  able  to  take  them  up  again  till  next 
spring.  I  believe,  however,  that  I  may  alieady  say  that  ozone  can  have  but  a 
vei-y  secondai-y  influence  on  the  phenomenon,  and  that  if  light  is  more  active 
within  the  same  length  of  time  at  the  north  than  in  Fiance,  the  I'eason  is  that  it 
has  lost  fewer  of  its  chemical  radiations  in  passing  through  the  atmosphere,  because 
the  latter  is  poorer  in  oxidizable  substances.  I  know,  of  course,  that  there  are  in  the 
north  those  pine  forests,  of  which  I  have  spoken  before,  and  that  perhaps,  if 
Helsingfors  were  built  in  the  heart  of  the  woods,  instead  of  being  a  large  city  on 
the  sea-coast,  tlie  [)oints  of  difference  would  be  somewliat  less.  But  there  would 
always  remain  the  fact  that  the  quantities  of  vapor  diffused  through  the  air 
increase  with  the  temperature,  and  that,  for  one  and  the  same  aspect  of  the  fauna 
and  the  floi'a  of  the  earth,  the  equatorial  atmosphere  will  always  be  more  heavily 
loaded  than  that  of  the  temperate  zones,  which  in  its  turn  will  again  be  more  so 
than  that  of  boreal  regions. 

Whatever  finally  the  explanation  of  the  fact  may  be,  the  impoitant  point  is  to 
show  that  it  exists,  and  that  there  is  a  difference  in  light,  so  far  as  its  quality  is  con- 
cerned, at  the  same  hours  of  the  da}^,  at  the  north  and  in  the  heart  of  Europe. 
But  this  is  not  all.  After  having  examined  this  question  of  quality,  we  have  to 
look  next  at  the  question  of  quantity.  The  days  which  are  useful  to  vegetation  at 
the  north  are  longer  than  with  us — what  now  is  the  influence  of  the  duration  of  light 
on  the  chemical  phenomenon  which  serves  us  as  a  means  of  measurement  ?  Is  the 
effect  thus  produced  proportionate  to  the  length  of  exposure  to  the  sun  ?  Does  it  in- 
crease more  or  less  slowly  ?    Such  are  the  fii'st  questions  which  we  have  to  consider. 

I  believe  they  are  new,  because  up  to  this  day,  both  as  I'egards  meteoiologic 
instruments  and  in  theoretical  speculations,  it  has  always  been  held  that  the  effect 


30  ATMOSPHERIC  ACTINOMETRY 

of  illumination  was,  everything  else  being  equal,  proportional  to  its  duration.  We 
shall  see  that  this  is  not  so,  and  that  the  effect  inci-eases  much  more  rapidly  than 
the  increase  of  time,  so  that  all  the  notions  which  we  entertain  on  this  subject 
stand  in  need  of  revision. 

INFLUENCE    OF   THE   DURATION    OF   ILLUMINATION. 

So  far  we  have  taken  as  a  measure  of  the  total  actinic  effect  during  the  period 
of  exposure,  the  sum  total  of  the  oxalic  acid  consumed.  The  conclusions  which 
we  have  thus  reached,  subsist,  whatever  may  be  the  law  which  connects  the  com- 
bustion with  the  actinic  effect ;  it  has  been  enough  for  us  to  expose,  duiing  the 
same  length  of  time,  solutions  equally  sensitive,  in  two  different  places  and  to  pro- 
ceed always  by  comparison. 

But  the  law  of  the  increase  of  actinic  effect,  with  the  time  of  insolation,  does 
not  the  less  merit  attentive  investigation.  To  begin,  let  us  ask  first,  if  the  total 
effect  of  combustion,  observed  at  the  close  of  a  day,  upon  a  solution  of  oxalic  acid 
exposed  to  the  sun,  represents  the  sum  of  the  divers  actinic  effects  produced  at  the 
different  hours. 

One  way  to  answer  the  question  is  to  expose  in  the  moi-ning,  side  by  side,  two 
vessels  holding  the  same  quantity  of  the  same  solution.  One  is  to  be  examined  at 
the  end  of  the  day,  and  this  will  give  us  the  sum  total  of  the  effect.  The  other  is 
to  be  examined  at  the  end  of  an  hour,  and  then  to  be  replaced  by  another  vessel  like 
the  first,  but  containing  new  liquid,  to  be  likewise  studied  after  another  hour's  ex- 
posure. In  like  manner,  we  shall  renew  the  study  at  successive  hours.  If  the 
actinic  effects  accumulate,  without  loss  or  encroachments  in  the  liquid  of  the  vessel 
which  has  been  exposed  to  the  sun  all  day  long,  the  quantity  of  acid  which  we 
shall  find  has  disappeared  must  equal  the  sum  of  the  quantities  of  acid  which  have 
vanished  in  the  vessels  that  were  exposed  for  an  hour  each. 

The  experiment,  repeated  again  and  again,  shows  that  it  is  never  so.  The  sum 
of  the  quantities  of  acid  burnt  in  the  vessels  which  have  been  exposed  each  one 
hour  only,  is  always  insignificant  in  comparison  with  that  consumed  in  a  vessel 
which  has  spent  the  whole  day  in  the  sunlight.  The  difference  vaiies  from  one  day 
to  another,  and  increases  with  the  intensity  of  combustion.  It  decreases  slightly  if 
we  carry  the  exposure  of  the  successive  vessels  to  two  hours,  and  still  more  if  we 
extend  it  to  three  or  four  hours,  as  should  be  expected.  But,  even  if  we  divide  a 
day  of  ten  hours  into  two  equal  periods,  one  fi'om  7  a.m.  to  noon,  and  the  other 
from  noon  to  5  p.m.,  the  sura  total  of  acid  consumed  in  the  two  vessels  that  correspond 
to  the  two  periods  of  exposure  sometimes  does  not  exceed  half  the  acid  consumed 
in  the  vessel  which  spent  ten  hours  in  the  sun.     The  combustion,  therefore,  does 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  31 

not  begin  as  soon  as  the  vessel  is  exposed  to  the  light.  There  is  a  "  lost  time  "  at 
the  beginning;  two  hours,  three  hours,  are  necessary  for  the  solution  to  put  itself 
in  action.  During  this  whole  time  the  work  is  wholly  interior  and  betrays  itself 
by  no  diminution  of  the  acidimetric  titre. 

This  "dead  time"  at  the  beginning  of  the  reaction  should  not  surprise  us. 
When  we  study  the  different  reactions  of  chemistry,  from  this  point  of  view,  we 
become  aware  that  there  are  few  which  begin  immediately  upon  realization  of  the 
exterior  conditions  of  production,  even  in  cases  when  the  energy  which  comes  into 
play  is  altogether  internal  as  regards  the  mixture.  The  formation  of  a  precipitate 
of  barium  sulphate  is  not  instantaneous  ;  that  of  calcium  sulphate,  or  of  calcium 
tartrate,  is  still  less  so  ;  a  mixture  of  formate  and  of  permanganate  of  potassium 
remains  apparently  inert  for  some  seconds,  after  which  begins  an  abundant,  and,  to 
some  extent,  an  explosive  libei'ation  of  carbonic  acid,  proceeding  from  the  combus- 
tion of  formic  acid. 

Here  the  heat  produced  by  the  reaction  intervenes  to  increase  its  activity.  We 
can  say  the  same  of  the  phenomenon  which  Bunsen  and  Roscoe  discovered  and  in- 
vestigated under  the  name  of  "photochemical  induction,"  in  the  combination,  in  the 
sunlight,  of  chlorine  and  hydrogen.  This  reaction  also  requires  a  cei'tain  time  to 
commence,  but  it  accelerates  afterwards,  because  it  is  exothermic.  The  same 
remark  applies  to  the  reduction  of  chloride  or  bromide  of  silver  in  the  presence  of 
an  organic  substance,  which  also  shows  a  "  dead  time  "  at  the  beginning,  and  be- 
comes more  energetic  afterwards.  The  same  remark  applies,  moreover,  to  almost 
all  photogi'aphic  operations,  whether  we  wish  to  obtain  luminous  impressions,  to 
develop  images,  or  to  produce  positive  prints. 

If  we  observe  a  "  dead  time  "  when  the  forces  are  internal  and  accelerating,  it  is 
not  surprising  that  we  should  find  a  like  one  also  in  the  solar  combustion  of  oxalic 
acid,  where  the  impulse  is  to  come  from  without  and  where  the  reaction  is  so  feebly 
exothermic.  But  this  verification  presents  here  an  interest  which  it  has  not  else- 
where, for  we  connect  it  intimately  with  the  phenomena  of  sensibilization,  which 
we  discovered  pi-eviously  in  the  solutions  of  oxalic  acid.  In  both  cases  a  molecular 
preparation  is  evidently  involved,  the  mechanism  of  which  is  still  unknown  to  us, 
but  which  results  in  placing  the  molecule  upon  a  kind  of  inclined  plane,  down 
which  it  may  be  made  to  roll  by  the  slightest  impulse.  As  a  confirmation  of  this 
idea,  I  have  ascertained  that  in  fact  the  "  dead  time  "  at  the  beginning  is  less  pro- 
tracted with  solutions  which  have  been  made  sensitive,  than  with  new  solutions,  so 
that  if  the  latter  do  not  undergo  in  the  light  of  the  sun,  as  we  have  seen  before, 
the  same  degree  of  combustion  as  the  othei's,  it  is  partly  because  the  "  dead 
time  "  at  the  beginning  is  shorter.     But  I  say  "  partly  "  because  there  is  still  another 


32  ATMOSPHERIC  ACTINOMETRY 

pbenomeuon.  We  shall  see  that  combustion,  once  begun,  does  not  go  on  with  I'egu- 
lar  anil  equal  ste[)s,  but  is  made  to  proceed  faster  and  faster.  In  other  words,  every- 
thing goes  on  as  if  the  sensitiv^eness  were  increasing  during  the  oxidation.  To  put 
it  still  differently,  the  quantity  of  burnt  oxalic  acid,  which  amounts  to  little  or  next 
to  nothing  during  the  first  moments  of  the  ex[)Osure  to  the  sun,  starts  out  and  in- 
creases, from  that  instant,  quicker  than  time,  so  that  there  is  no  pi'oportion  between 
the  length  of  the  insolation  and  its  consuming  effects. 

In  order  to  take  account  of  effect  of  insolation  upon  an  oxalic  solution,  let  us 
slightly  modify  the  conditions  of  an  experiment  which  I  have  Just  described.  Let 
us  expose  in  the  morning  a  dozen  similar  vessels  to  the  sun,  and  let  us  withdraw 
every  other  hour  two  of  them,  which  will  give  us  the  sum  total  of  combustion  up  to 
that  moment.  It  will  be  easy  by  this  means  to  ascertain  the  progress  of  combustion 
during  the  whole  day.  The  following  experiment  I  cite,  not  as  being  the  most 
complete  of  those  which  I  have  made,  but  because  it  was  pei-forined  with  a  solution 
of  the  same  sensitiveness  as  that  used  in  other  experiments  which  I  shall  (.pote 
presently,  so  that  all  of  them  are  compai-able. 

Ex]p. — -On  September  6,  1888,  at  8.30  a.m.,  on  Mont  Dore,  I  exposed  to  the  sun 
four  vessels,  which  I  withdrew  at  various  intervals,  and  in  which  I  meas- 
ured the  proportion  of  oxalic  acid  burnt. 

Solar  Combustion  % 

After  2  hours  o  ^ 

4  3  ^  ^- 

"     8     "  lo  s^ 

"     10       "  12    <^ 

We  see  at  the  start  the  "  dead  time  "  of  the  beginning.     We  see,  moreover, 
that  from  the  fourth  to  the  eighth  hour,  that  is  to  say  from  12.30  p.m.  to 
4.30  P.M.,  the  combustion  was  twice  as  I'ajjid  as  from  10.30  a.m.  to  noon,  in 
spite  of  the  gi-adual  descent  of  the  sun  towards  the  horizon.     During  the 
last  two  hours,  and  notwithstanding  the  obliquity  of  the  solar  rays,  which 
is  already  great  at  this  time  of  the  year,  the  combustion  was  still  two 
thirds  of  what  it  had  been  between  10  a.m.  and  noon. 
It  is  always  the  same,  whether  the  total  combustion  be  feeble,  as  it  was  at 
Mont  Dore,  or  active,  as  I  have  at  times  found  it  in  Paris.     Fi'om  the  sum  total  of 
my  results  I  think  I  may  conclude  that  the  progress  of  solar  combustion  does  not 
remain  constant  during  the  whole  of  the  day,  and  that  instead  of  increasing  towards 
noon,  in  oi'der  to  decrease  afterwards  in  proportion  as  the  sun  appi'oaches  the  hori- 
zon, it,  on  the  contrary,  experiences  a  progi-essive  acceleration  which  does  not  cease 
till  the  sun  is  near  its  setting. 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  33 

Everything,  then,  goes  on  as  if  the  sensitiveness  which,  as  we  have  seen,  a  solu- 
tion of  oxalic  acid  attains  if  left  to  itself  in  darkness,  were  not  by  any  means  a 
maximum  sensitiveness,  and  might  be  greatly  increased  in  the  light.  I  have  in  fact 
ascertained — and  we  shall  presently  see  an  example  of  it — that  a  recent  solution  of 
oxalic  acid  may  be  made  very  sensitive  by  a  few  hours'  exposure  to  the  sun,  and  so 
be  brought  up  to  the  level  of  an  old  solution,  or  even  beyond  it.  But  then  an  un- 
foreseen consequence  appears :  the  sensitiveness  acquired  in  darkness  is  persistent 
— might  it  not  be  perhaps  the  same  with  sensitiveness  acquired  in  the  light,  so  that 
the  accelerating  effect  of  a  fine  day  might  spread,  as  it  were,  in  its  totality,  or  at 
least  in  part,  over  the  following  day  ? 

The  following  experiment  proves  in  fact  that  a  solution  left  for  a  whole  day  in 
the  sun,  and  which  has  not  been  entirely  oxidized,  will  retain  for  the  next  day  a 
greater  sensitiveness  than  another  part  of  the  same  solution  which  was  not  pre- 
viously insolated. 

Exp. — There  were  exposed  every  day  to  the  light  four  identical  vessels,  two  of 
which  were  carefully  examined  at  the  close  of  each  day,  while  the  other 
two  were  left  in  leserve  foi-  the  day  following;  on  this  day  they  were 
again  exposed  to  the  sun  at  the  same  time  with  the  four  new  vessels  of  a 
second  experiment.  The  total  of  combustion  in  the  vessels  which  were 
exposed  for  two  days  was  then  compared  with  the  sum  of  combustions  in 
the  vessels  which  wei'e  each  exposed  only  one  day.  Some  of  the  results 
which  I  thus  obtained  are  as  follows : 

The  day  of  Sei)tember  2d,  combustion  iQ%)        j  ■      ,, 

3d,  24  f  ) 

Both  days  together,  68  ^. 

The  amount  of  combustion  has  therefore  doubled.     Here  are  the  results 
of  another  experiment : 

The  day  of  September  4th,  combustion  12  ^  )         ,  -      ,, 

"   "  '•      "     sth,      "      II  ^r^^ '"''"• 

Both  days  together,  "  38  i. 

The  diffei'ence  points  in  the  same  dii'ection  as  the  preceding  experiment ;  only 
it  is  not  quite  as  great,  because  the  two  days,  September  4th  and  5th,  were  both 
quite  indifferent  days  (page  28),  whilst  the  day  of  September  3d  in  the  first  experi- 
ment was  very  fine. 

To  sum  up,  we  see  that  the  insolated  vessel  of  the  first  experiment  under- 
went on  the  second  day  a  combustion  of  68  -  10  =■  58  per  cent,  while  a  new  vessel 
suffered  only  an  oxidization  of  24  per  cent.     As  to  the  second  experiment,  the  cor- 


34  ATMOSPHERIC  ACTINOMETRY 

responding  figures  are  26  per  cent  and  11  per  cent;  this  shows  that  not  only  does 
the  sensitiveness  of  the  oxalic  solution  increase  in  consequence  of  insolation,  but  the 
gain  continues  during  the  night.  Some  experiments  of  the  same  kind,  which  I  will 
not  now  explain  in  detail,  prove  that  this  excitation  of  sensitiveness  by  means  of  inso- 
lation, endures  even  to  the  second  day  after,  in  a  solution  which  is  kept  in  the  dark 
after  having  been  exposed  for  a  day  to  the  sun.  It  is  only  after  three  days,  there- 
fore, that  traces  of  sensitiveness  are  no  longer  discernible.  By  that  time,  the 
insolated  solution  has  nearly  i-eturned  to  the  degiee  of  sensitiveness  which  the 
mother  liquid  possessed,  which  seems  thus  to  coi-respond  to  a  kind  of  equilibrium. 
It  is  in  fact  remarkable  that  the  diiferent  sensitive  licpiids  which  I  have  used  in  my 
long  experiments  and  which  were  prepared  at  very  different  times,  with  the  single 
precaution  that  they  were  not  to  be  used  before  the  lapse  of  several  months,  had  all 
of  them,  at  the  moment  when  I  used  them,  very  nearly  the  same  sensitiveness.  It 
was  on  September  8th  and  9th  that  I  had  to  change  my  solutions  at  Mont  Dore, 
and  I  availed  myself  of  the  fact  that  these  two  days  wei'e  but  indifferently  fair,  to 
interrupt  my  sei'ies  and  to  compare  again  and  again  the  old  liquid  with  the  new. 
The  titre  was  always  the  same  for  both.  Mr.  Elfving  compared  likewise  two  solu- 
tions which  I  had  sent  him  a  year  apart,  and  found  in  four  days  of  experimenting 
the  following  corresponding  figures  for  the  old  and  the  new: 


Old  Solution. 

New  Solution, 

I  St  day,  combustion, 

58^ 

s^i 

2d       " 

52^ 

5ii 

3d     " 

635^ 

60^ 

4th    " 

42  X 

35  ^ 

The  old  solution  was  a  little  more  sensitive,  which  is  the  usual  rule.  But 
the  difference  was  trifling,  and  thus  our  former  statement  was  confirmed  (page  9). 
The  oxalic  solution,  kept  in  diffused  light,  reaches  a  fairly  constant  sensitiveness  in 
a  few  weeks,  but  this  maximum,  although  stable,  is  not  a  maximum  maximorimi. 
It  may  be  temporarily  exalted  in  the  sun,  continue  if  the  illumination  continues, 
and  return  to  its  original  level  after  some  days  of  darkness. 

In  order  to  make  this  conclusion  really  valuable,  we  have  to  overcome  one  last 
objection.  Might  it  not  happen  that  the  increase  of  combustion  discovered  on  the 
second  day  in  a  vessel  which  had  been  insolated  on  the  day  before,  might  mean 
simply  the  suppression  of  the  "dead  time"  at  the  beginning  ?  Starting  earlier,  the 
combustion  might  better  utilize  the  good  hours  of  the  day,  and  thus  be  enabled  to 
go  farther.  A  priori,  the  intervention  of  this  cause  does  not  seem  to  explain  suffi- 
ciently the  great  difference  observed.    But  it  is  safer  to  consult  experience.    It  will  be 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  85 

sufficient  to  cut  the  two  days  of  observation  by  an  intermediate  examination  and  to 
discover  the  intensity  of  combustion  during  each  one  of  the  intervals.  Thus  it  can 
easily  be  seen  whether  the  vessels  change  at  the  same  rate,  after  having  recovered 
their  "  dead  time,"  or  whethei-  the  vessel  which  had  been  insolated  on  the  previous 
day,  still  progi-esses  more  rapidly  than  its  neighbor? 

ISayp. — On  Septeniber  12,  1888,  a  fine  day  with  a  warm  sun,  and  a  few  cumuli, 
there  wei'e  exposed  to  the  sun  4  vessels,  Nos.  I,  2,  3,  and  4. 
Vessel  1  was  examined 

after  5  houi's  ;  combustion  10  per  cent. 
Vessel  2 

after  9  hours;  combustion  18  percent. 
Vessels  3  and  4  were  put  aside  and  exj)osed  anew  on  the  next  day  with 
two  new  vessels,  Nos.  3'  and  4'.     This  day,  the  13th,  was  very  fine,  with  a 
few  cirri  in  the  morning.   It  changed  a  little  for  the  worse  towards  evening. 
Vessel  3'  was  examined 

after  5  hours;  combustion  13  per  cent. 
Vessel  3 

after  5  hours ;  combustion  44  per  cent. 
The  difference  is  considerable  and  is  certainly  in  part  at  least  due  to  the 
suppression  of  the  "  dead  time  "  in  the  vessel  which  was  insolated  on  the 
day  before.     But  this  again  is  not  all,  for  during  the  second  half  of  the 
day,  the  insolated  vessel  kept  up  a  much  more  rapid  progress  than  the 
other,  as  the  following  figures  clearly  show  : 
Vessel  4'  examined  after  9  hours ;  combustion  25  per  cent. 
Vessel  4  examined  after  9  hours ;  combustion  62  per  cent. 
The  acceleration  in  the  solution  which  had  been  insolated  on  the  day 
before,  thus  continued  throughout  the  Jay,  and  while  in  the  second  half 
of  the  second  day,  the  new  liquid  only  showed  an  increase  of  25  —  13  = 
12  per  cent  in  its  combustion,  the  liquid  insolated    the    day  before  rose 
from  44  to  62,  undergoing  thus  an  increase  of  18  per  cent. 
It  will  also  be  noticed  that  in  the  morning  this  same  liquid  had  increased 
from  44  —  18  =  26  per  cent,  while  the  new  liquid  experienced  a  combustion  of  only 
13  per  cent.     Here  has  come  in  the  double  effect  of  suppression  uf  "dead  time" 
and  that  of  the  accelei'ation.     The  two  solutions  V)ecame  a  little  more  nearly  equal 
towards  evening,  but  the  insolated  solution  continued  its  quicker  progress. 

Thus  there  can  be  no  doubt  that  the  insolation  during  the  previous  day 
continued  its  effects  over  the  next  day  and  the  day  after  that.  But  this  is  not  all. 
One  fact,  no  less  curious  than  the  preceding,  is  that  the  sensitiveness  due  to  the 


36  ATMOSPHERIC  ACTINOMETRY 

action  of  light,  enables  the  solution  to  undergo  in  diffused  light  a  combustion 
which  is  out  of  question  as  long  as  it  has  only  its  normal  degree  of  sensibility,  that 
is,  the  degree  obtained  by  keeping  for  some  weeks  in  a  diffused  light. 

Ea^. — In  August,  1889,  at  Noalhac  near  Aurillac  (Cantal),  at  an  altitude  of 
about  700  metres,  I  prepai'ed  a  solution  of  oxalic  acid,  part  of  which  was 
left  in  a  flask,  exposed  to  a  very  feeble  light,  while  another  portion  was 
exposed  to  the  sun  in  a  stoppered  bottle.  This  was  in  order  to  see  if  the 
process  of  i-endei-ing  the  solution  sensitive  was  necessai'ily  accompanied 
by  a  pi'ocess  of  combustion,  or  whether  it  could  be  accomplished  without 
the  latter.  The  experiment  showed  that  the  two  effects  are  independent 
of  each  other.  The  solution  contained  in  the  closed  flask  had  at  its  dis- 
posal only  a  very  small  quantity  of  dissolved  oxygen,  which  it  consumed, 
moreover,  with  but  a  slight  diminution  of  its  titre,  but  in  one  day  of  inso- 
lation it  I'eached  an  intense  sensitiveness,  which  was  maintained  for 
seveial  days  at  the  same  rate  by  preserving  it  in  a  diffused  light. 

On  August  30th,  I  exposed  to  the  sun  two  vessels  containing  some  of 
this  insolated  solution,  and  at  the  same  time,  two  vessels  of  the  same 
non-insolated  preparation.     The  figures  for  solar  combustion  are  : 
18  and  19  per  cent  for  the  non-insolated,  and 
92  and  92        "  "      "    insolated  liquid. 

The  day  was  a  very  fine  one;  the  flask  with  the  insolated  solution 
remained  in  the  sun,  but  closed.  On  August  31st,  two  groups  of  two 
vessels  each  were  prepared,  one  containing  non-insolated,  the  other  inso- 
lated solution.  One  of  these  groups  was  exposed  to  the  sun,  the  other 
on  a  window-ledge,  facing  the  north,  wheie  it  received  no  light  but  that 
coming  from  the  sky  and  dimmed  somewhat  by  a  slight  dry  fog.  The 
figui'es  found  after  8  hours'  exposui'e,  for  combustion  in  the  sun  and  in 
diffused  light,  were  the  following: 

Insolated  liquid  lost  63  per  cent  in  the  sun. 

Non-insolated      "         "     24         "         "     "      " 
Insolated  "         "19         "        in  diffused  light. 

Non-insolated      "        "       6 
The  next  day,  September  1st,  dui'ing  a  dark,  threatening  day,  the 
same  arrangement  gave  the  following  results : 

Insolated         liquid  lost  50  per  cent  in  the  sun. 
Non-insolated      "         "     13         "         "     "      " 
Insolated  "         "      6         "        in  ditt'used  light. 

Non-insolated      "        "      3         "         "         "  " 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  37 

Thus  a  previous  insolation  increases  the  rapidity  of  combustion,  not  only  in  di- 
rect light  but  also  in  diffused  light.  The  experiment  of  September  2d  shows,  how- 
ever, when  compared  with  that  of  the  day  before,  that  this  diffused  light  must 
have  a  certain  intensity  to  make  its  effect  measurable  after  some  hours.  But, 
viewed  by  itself,  this  experiment  shows  that  even  a  dark  day  still  has  an  accelerat- 
ing effect  upon  a  solution  which  has  not  seen  the  sky  since  the  second  day  before. 
The  insolated  liquid  was  from  that  moment  kept  in  diffused  light  at  the  back 
of  a  room  with  but  one  window,  facing  the  north.  It  was  found  that  after  a  few 
days  it  had  not  sensibly  changed  in  titre,  l>ut  variation  began  to  show  itself  at  the 
end  of  a  month.  We  tlius  see  how  here  also,  in  spite  of  good  conditions  of  preser- 
vation,  the  phenomena  of  slow  combustion  appear  which  have  been  observed  since 
Wettstein  in  solutions  of  oxalic  acid.  Not  insolated,  this  solution  preserved  in  the 
same  manner  had  remained  much  more  stable,  which  shows  that  it  is  necessary  to 
avoid  exposure  to  light,  even  temporarily,  or  even  in  a  carefully  closed  flask,  of 
solutions  of  oxalic  acid  which  are  intended  for  processes  of  titration.  The  lumi- 
nous impression,  once  received,  persists  and  makes  them  much  less  stable — it  con- 
tinues, as  we  shall  presently  see,  even  after  the  liquid  solution  has  been  placed  in 
darkness. 

Eocfp. — Another  experiment  was  begun  identical  with  those  that  have  just  been 
described  except  that  the  flask  which  contained  the  insolated  solution, 
sheltered  from  the  air,  was  kept  for  three  nights  and  two  days  in  a  cupboard 
of  th&  laboratoiy  before  being  distributed  into  vessels  on  September  6th. 
Unfortunately  the  day  of  the  6th  was  disturbed  by  cirri  and  cloudlets. 
The  insolated  liquid  Tost  20  per  cent  in  the  sun, 
Non-insolated  liquid  lost  7  per  cent  in  the  sun. 
•    The  oxidation  in  diffused  light  was  msignificant. 
The  proportion  of  oxalic  acid  bm-nt  in  sunlight  is,  therefore,  still,  after  60 
hours  of  obscurity,  tliree  times  greater  in  the  insolated  solution  than  in  the  other. 
But  the  sensitiveness  decreases  afterwards  and  the  difference  soon  ceases  to  be 
measurable  after  a  day's  exposure.     We  here  meet  again  with  that  retrogradatiou 
which  we  liave  pointed  out  earlier,  and  which  brings  us  back  to  nonnal  sensitive- 
ness. 

I  add,  in  order  to  close  the  subject,  that  this  solar  impression,  which  disap- 
pears slowly,  is  on  the  other  hand  produced  veiy  rapidly,  and  that,  when  investi- 
gating comparatively,  vdth  respect  to  the  combustion  which  they  undergo,  the 
solution  which  I  had  exposed  to  the  sun,  in  three  flasks,  and  1,  2  and  3  days, 
respectively,  I  have  not  been  sible  to  show  that  there  was  any  essential  difference 
between  them  ! 


38  ATMOSPHERIC   ACTINOMETRY 

I  have  left  aside,  in  all  which  precedes,  the  question  of  the  mechanism  con- 
nected Avith  both  sensitization  and  combustion.  The  former  goes  t)n  when  the 
solution  is  sheltered  from  the  air,  and  can  take  place  only  by  a  new  ari'angement 
of  molecules.  Combustion,  on  the  other  hand,  takes  place  in  contact  with  the  air 
and  possibly  with  the  formation  of  ozone  or  of  hydrogen  peroxide.  That  is  a  ques- 
tion which  must  be  investigated  by  itself.  1  purpose  here  only  to  put  in  evidence, 
as  regards  the  constitution  of  the  atmosphere,  some  properties  and  a  varial>ility  of 
effects,  not  hitherto  observed. 

Metekeologioal,  Hygienic,  and  Agricultural  Effects. 

If  we  now  return,  with  the  results  which  we  have  obtained,  to  our  investiga- 
tion of  the  causes  which  provoke  the  rapid  development  of  vegetaticm  in  the  exti'eme 
northern  regions,  we  see  that  those  regions  are  superior  to  ours  in  a  twofold 
aspect. 

1.  That  cause  which  depends  on  the  constitution  of  their  atmosphere  consists  in 
this,  that  the  absoi-ption  of  the  chemical  radiations  of  solar  light  is  there  less  great 
than  with  us.  The  actinic  power  at  the  level  of  the  soil  exceeds  that  which  we 
have  obsei-ved  around  ourselves  at  different  hours  of  the  day,  and  that  in  spite  of 
a  lower  sun  and  a  greater  thickness  of  atmosphere,  which  the  rays  must  traverse. 
These  differences  are  due  mainly  to  the  fact  that  vegetation  in  the  north  sends 
into  the  air  fewer  oxidizable  substances  to  form  a  screen. 

2.  The  other  point  of  superiority  connected  with  the  geographical  situation 
consists  in  this,  that  in  the  extreme  north,  during  the  })eriod  of  vegetation,  the 
days  are  longer  than  in  our  temperate  zones,  and  that  the  actinic  power,  at  least 
so  far  as  it  may  be  measured  by  a  solution  of  oxalic  acid,  increases  more  rapidly 
than  the  length  of  the  day,  and  this  out  of  all  proportion.  After  a  pei'iod  of  prepa- 
ration, combustion  })egins,  then  accelerates  so  rapidly  as  to  make  up  for  time  lost 
at  the  beginning,  and  finally,  towards  evening,  reaches  unusually  high  figures,  such 
as  are  unknown  to  our  regions.     It  is  in  this  way  that  combustion  has  risen  to 

37  and  89  per  cent  on  September  2  and  4,  1887, 
79  and  80  per  cent  on  September  9  and  11,  1888, 
75  per  cent  on  Aug.  27,  1888  at  Helsingfors. 

and  this  at  a  time  when  the  highest  figures,  relatively  to  the  same  periods  and  with 
the  same  solutions,  did  not  exceed  50  per  cent  and  were  even  sometimes  much 
lower  in  our  country.  In  order  to  reach  figures  equal  to  those  obtained  in  the  Gulf 
of  Finland,  it  was  necessary  for  me  in  France  to  accumulate  upon  my  vessels  the 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  39 

radiation  of  two  consecutive  days.  It  may  thus  be  said  that,  speaking  generally,  a 
day  at  the  north  is  worth  two  of  ours  as  regards  actinic  power. 

Not  only  does  the  actinic  effect  of  a  fine  day  increase  more  rapidly  than  the 
length  of  the  day  itself,  but  it  may  actually  spread  itself  over  the  next  day,  and 
the  day  after  that,  and  thus  make  up,  in  some  degree,  for  the  absence  of  the  sun. 
In  like  manner,  a  fine  morning  may  render  combustion  more  rapid  even  though 
the  evening  be  dark  and  stonuy.  It  is  enough  that  the  liquid  shall  have  been 
made  sensitive;  and  as  this  sensitization  is  the  more  rapid  as  the  actinic  intensity  is 
greater,  the  atmospheric  condition  of  northern  countries  favors  them  in  this  respect 
beyond  us,  and  a  new  superiority  is  thus  attained  through  the  superposition  and 
mutual  emphasis  of  the  other  two  causes. 

Finally,  the  sensitiveness  produced  by  a  fine  day  continues  for  several  days. 
A  number  of  bad  days,  following  each  other,  is  consequently  not  a  period  of 
inertness  and  loss ;  it  draws  upon  the  store  which  was  collected  during  fine 
weather.  On  the  other  hand,  we  have  seen  that  the  sensitiveness  which  was 
acquired  in  the  sun,  did  not  increase  without  limit,  and  that  it  reached  quite  rap- 
idly a  maximum  beyond  which  it  did  not  go.  A  succession  of  fine  days,  therefore 
does  not  develop  actinic  phencmiena  to  an  extreme.  We  here  meet  once  more 
with  the  system  of  balancing  which  weakens  gi'eat  effects,  increases  small  ones, 
and  which  has  been  i)ointed  out  with  regard  to  so  many  other  manifestations  of 
the  forces  of  nature. 

Summing  up  the  matter,  then,  it  would  seem  that  we  have  hitherto  missed  our 
way,  in  considering  the  chemical  action  of  solar  light  as  independent  of  locality  and 
proportionate  to  time  of  isolation,  or  as  furnished  or  measured  by  meteorological 
instruments.  The  first  of  these  notions  was  purely  instinctive  and  was  suggested 
especially  by  the  imiformity  which  was  ascertained  to  exist  at  different  points  of 
the  globe  in  so  many  other  gi'and  meteorological  phenomena  (such  as  the  composi- 
tion of  the  air,  the  average  barometric  elevation,  the  mean  distribution  of  nebulos- 
ity, etc.)  Instead  of  such  a  uniformity,  we  find,  on  the  contrary,  actinic  climates, 
limited  in  point  of  surface,  for  they  betray  the  local  influence  of  the  surface  of  the 
soil — limited  also  in  point  of  duration,  for  they  are  due  to  two  kinds  of  clouds 
which  are  subject,  like  the  others,  to  the  influences  of  place  and  season. 

Misjudging  thus  local  influences,  only  the  first  cause  has  been  thought  of,  and 
all  efforts  had  been  directed  towards  measuring  the  duration  of  insolation.  On 
this  point,  I  think  I  have  shown  that  the  wrong  way  had  been  taken.  The  actinic 
force  of  a  day  is  not  the  same  for  the  same  day,  in  different  parts  of  the  globe,  and 
its  effect  increase8"~hiore  rjipidly  than  its  length ;  such  is  the  principal  lesson  of  this 
Memoir. 


40  ATMOSPHERIC  ACTINOMETRY 

One  step  farther  might  be  taken.  We  have  just  ascertained  that  in  the  solu- 
tion of  oxalic  acid  there  takes  place  a  kind  of  storing  up  of  light,  which  shows 
itself  in  an  increase  of  sensitiveness  as  regards  phenomena  of  oxidation.  Might 
not  the  oxygen  which  is  present  in  the  solution,  or  even  that  which  is  constantly 
dissolved  there  and  transformed,  might  it  not  itself  be  rendered  sensitive,  and  so  be 
endowed  with  an  oxidizing  power  which  it  could  afterwards  use  in  diffused  light  ? 

I  have  found  nothing,  while  searching  in  this  direction,  with  oxalic  acid  ;  this 
reagent,  quite  sensitive  enough  for  the  study  of  powerful  actions,  is  not  sensitive 
enough  for  such  weak  actions  as  that  which  I  have  just  suggested.  But  I 
have  been  more  successful  with  oxidizable  substances  of  sharper  reactions,  so  that 
the  very  smallest  variations  became  measurable.  This  is  the  case  with  diastases ; 
an  almost  infinitesimal  quantity  will  produce  very  apparent  effects,  and  it  is,  there- 
fore, easy  to  trace  their  disappearance  by  oxidation  in  the  liquids  which  contained 
them.  With  rennet  especially  the  very  smallest  variations  in  quantity  can  be  a[)- 
preciated  from  corresponding  variations  in  the  time  of  coagulation  of  equal  quan- 
tities of  milk,  so  that  this  diastase  is  very  convenient  for  study.  By  sitch  means 
I  found  that  it  oxidized  and  disappeared  in  water  which  had  been  previously 
exposed  to  the  sun,  while  it  remained,  if  not  quite  intact,  at  least  nearly  so  in  the 
same  water  as  freshly  drawn  from  the  hydrant.  I  also  found  that  a  glass  flask 
exposed  to  the  sun  stored  up  on  its  walls  enough  chemical  radiations  to  accelerate 
afterwards  the  oxidation  of  a  solution  of  rennet,  which  was  allowed  to  stand  in  the 
shade. 

All  these  facts,  upon  which  I  do  not  insist  because  they  go  beyond  the  limits 
of  this  work,  enlarge  the  field  of  those  phenomena  to  which  they  apply.  If  inso- 
lated  liquids  and  solids,  may  in  certain  cases,  like  our  solutions  of  oxalic  acid, 
acquire  properties  which  they  had  not  before,  the  phenomena  of  solar  combustion 
may  well  extend  below  the  surface,  which  has  been  directly  illuminated,  and  assume 
in  the  general  economy  of  the  world  an  importance,  no  doubt  as  yet  inferior  to 
that  of  microscopic  organisms,  but  certainly  no  longer  to  be  neglected,  as  it  has 
been  heretofore. 

Since  I  have  entertained  the  views  which  I  have  developed  in  the  preceding 
pages,  I  have  investigated  especially  their  agricultural  and  hygienic  consequences. 

As  far  as  hygiene  is  concerned,  I  have  shown  more  clearly  I  think  than 
Messrs.  Downes  and  Blunt  that  solar  light  kills  the  germs  of  microbes  suspended 
in  the  air.  I  have  proved,  moreover,  that  this  destruction  was  preceded  by  a 
veritable  attenuation} 

'  Annates  de  Chimie  et  de  Physique,  6tli  ser.,  vol.  v.,  1885. 


AND  THE   ACTINIC   CONSTITUTION   OF  THE  ATMOSPHERE.  ,  41 

I  have  ascertained  since  that  this  destruction  and  this  attenuation  go  on  in 
the  supei-ficial  layers  of  the  soil  and  even  down  to  some  depth.  If,  in  the  many 
attempts  to  count  the  microbes  of  the  soil,  it  has  so  often  been  found  that  the 
number  is  less  near  the  surface  than  at  the  depth  of  a  few  centimetres,  we  must 
attribute  this  result  much  rather  to  solar  combustion  than  to  desiccation.  The 
sanitaiy  action  of  oxygen,  which  is  pursued  and  completed  in  the  atmosphere, 
begins  therefore,  thanks  to  light,  at  the  surface  of  the  soil,  and  the  healthiest  coun- 
tries are  those  in  which  the  actinic  power  of  the  sun  is  greatest. 

By  a  curious  mechanism,  which  I  have  tried  to  make  generally  known,  the 
solar  action  which  neutralizes  the  microbes  which  it  encounters,  can  act  like  them, 
and  take  their  place.  I  have  in  fact  shown  in  an  extensive  work  '  that  the  changes 
which  carbohydrates  undergo  upon  exposure  to  sunlight  are  exactly  like  those 
which  they  undergo  under  the  action  of  ferments.  Starting  from  the  same  point, 
these  two  modes  of  transformation,  apparently  so  different,  resemble  each  other 
not  only  in  their  variety  and  marvellous  flexibility  of  conduct,  but  still  more  in 
their  intermediary  and  final  products. 

Thus,  invert  sugar  in  alkaline  solution,  oxidized  in  sunlight,  gives  intermediary 
products  which  are  colloidal  and  identical  with  humic  acids,  except  that  they  are 
not  nitrogenous.  These  black  acids  are  afterwards  consumed  by  light,  exactly 
as  we  see  in  the  bleaching  of  the  black  soil  which  the  spade  or  the  plow  has 
turned  up. 

The  extreme  terms  of  the  transformation  of  this  sugar  or  of  its  humic  deriva- 
tives are  as  numei-ous  and  as  varied  in  solar  combustion  as  when  produced  by  the 
action  of  ferments.  Thus,  by  contact  with  potash,  or  with  soda,  we  obtain  alcohol 
through  an  interior  combustion  which  is  identical  with  that  produced  during  alco- 
holic fermentation.  On  the  other  hand,  in  the  presence  of  baryta,  no  alcohol,  but 
lactic  acid  is  produced.  In  this  there  is  analogy,  not  with  alcoholic  fermentation, 
but  with  lactic  fermentation,  and  this  analogy  is  all  the  closer  since — as  is  recog- 
nized— there  may  be  several  lactic  acids  of  different  rotaiy  power,  which  may  be 
produced  as  well  by  the  action  of  light  as  by  that  of  fenuentation. 

This  solar,  lactic  fermentation  is  accompanied  by  the  production  of  acetic  acid, 
as  in  the  case  of  microbian  fermentation.  In  other  cases,  butyiic  acid  is  formed, 
formic  acid,  oxalic  acid — in  short,  all  the  ordinaiy  residues  of  the  fennent  action. 
Finally,  carbonic  acid  represents  in  all  cases  the  extreme  tenn  of  the  change  of 
organic  matter  into  gas. 

The~luminous  action,  varying  in  quantity  according  to  place  and  season,  as  the 

'  Antiales  de  I'lnstiiut  Agronomique,  vol.  x.,  1886. 


42  ATMOSPHERIC  ACTINOMETRY 

different  chapters  of  this  Memoir  have  shown,  may  therefore  differ  in  the  (quality 
of  the  effects  which  it  produces.  All  these  facts  lend  to  the  study  of  chemical 
radiation,  an  importance  of  the  highest  rank,  and  I  shall  consider  myself  very 
happy,  if  the  first  results  contained  in  this  paper  shall  lead  men  of  science  to 
new  researches. 

SUMMARY. 

1.  The  oxidation  of  oxalic  acid  in  a  weak  solution  takes  place  mainly,  and 
almost  exclusively,  under  the  influence  of  the  chemical  rays  of  solar  light ;  it  can, 
therefore,  be  used  as  an  actinometric  measure. 

2.  It  depends  on  the  concentration  of  the  liquid,  which  for  the  best  results 
should  not  exceed  about  three  grammes  per  litre. 

3.  With  an  equal  volume  of  solution,  combustion  decreases  as  depth  increases ; 
there  is  an  absorption  of  chemical  rays,  although  the  liquid  is  and  remains  very 
transparent. 

4.  For  equal  depths  of  liquid,  combustion  is  proportional  to  the  surface,  and 
consequently  also  to  the  volume. 

5.  It  depends  on  the  age  of  the  solution,  that  is  to  say,  of  the  time  which 
has  elapsed  since  preparation.  As  it  grows  older,  an  oxalic  solution  becomes  more 
sensitive,  and  attains  a  certain  niaxiiuuin  which  is  (|uite  stal)le  and  (juite  regidar. 
It  is  well  to  wait  till  this  state  of  sensitiveness  has  been  produced. 

6.  The  daily  combustion,  such  as  is  measured  with  sterilized  liquids,  varies 
from  one  day  to  another  much  more  than  any  other  meteorological  phenomenon, 
and  while  subject  to  the  influence  of  what  we  call  "  fine  weather  "  and  "  overcast 
weather,"  it  manifests  very  clearly  other  influences  which  are  less  visible. 

7.  It  shows  also  the  influence  of  the  seasons,  and  manifestly  exhibits  a  maxi- 
mum in  spring. 

8.  It  is  but  feebly  subject  to  the  influence  of  altitude. 

9.  On  the  other  hand,  it  betrays  so  strongly  the  presence  of  divers  oxidizable 
essences  or  substances  in  the  air,  that  we  must  consider  local  and  daily  variations 
as  due  to  the  presence  in  the  atmosphere  of  actinic  clouds,  which  are  discoverable 
only  by  the  reduction  and  absorption  which  they  produce  in  the  chemical  radiations 
of  sunlight. 

10.  The  atmosphere  of  extreme  northern  regions  is  less  absorbent  than  that 
of  our  temperate  zones,  and,  consequently,  at  the  same  hours  of  the  day,  actinic 
radiation  is  more  powerful,  at  the  level  of  the  soil,  in  the  north  than  at  the  centre 
of  Europe. 


AND  THE  ACTINIC  CONSTITUTION  OF  THE  ATMOSPHERE.  43 

11.  Northern  countries  add  to  this  cause  of  superiority,  which  they  owe  to 
the  constitution  of  their  atmosphere,  another,  which  is  due  to  their  geographical 
position,  namely  :  that  the  actinic  effect  of  the  sun  increases  more  rapidly  than  the 
duration  of  its  presence  above  the  horizon.  The  veiy  long  days  of  the  north, 
during  the  period  of  vegetation,  are,  therefore,  in  their  actinic  effect,  more  active 
than  an  equal  number  of  days  in  our  temperate  regions,  and  we  can  thus  explain 
the  particularly  intense  rate  of  the  progress  which  vegetation  makes  in  the  vicinity 
of  the  polar  circle, 

12.  This  increase  of  sensitiveness  which  oxalic  acid  experiences  in  the  sun, 
does  not  cease  when  the  light  begins  to  fade,  and  may  continue  several  days. 
Hence  follows  a  conclusion  which  may  also  be  applied  to  our  temperate  regions: 
this  is,  that  the  actinic  effect  of  a  number  of  fine  days  in  succession  increases  more 
rapidly  than  its  duration,  and  also,  that  the  effect  of  a  fine  morning  is  not  lost  by  a 
dark  and  cloudy  evening. 

13.  We  must,  theiefore,  give  up  the  hope  of  finding,  in  the  duration  of  a  day 
or  of  solar  action,  a  measure  of  its  effects,  and  meteorological  instruments,  which 
accept  such  a  proportionality,  ai'e  to  be  rejected. 

14.  The  importance  of  these  actinic  phenomena  in  the  general  economy  of 
the  world  is  great  enough  to  make  it  necessary  that  we  should  approach  the 
investigation  by  appropriate  means. 


SUPPLEMENT. 

OBSERVATIONS  MADE   IN   1894   IN   FRANCE   AND    ALGIERS. 

Since  sending  my  Memoir  on  Atmospheric  Actinometry,  I  have  been  enabled 
by  the  courtesy  of  M.  Gessard,  Chief  Pharmacist  of  the  Military  Hospital  at  Setif 
(Algeria),  to  make  a  number  of  combined  obsei'vations  in  a  temperate  region  and  in 
a  hot  climate.  It  was  interesting  to  discover  whether  we  would  meet  here  with 
the  same  diffei'ences  as  between  the  obseivations  made  in  France  and  in  Finland, 
that  is  to  say,  if  for  equal  lengths  of  insolation  the  chemical  activity  of  the  solar 
rays  would  continue  to  diminish  in  proportion  as  we  approach  the  equator,  and 
as  their  calorific  power  increases. 

For  such  a  comparison  the  choice  of  the  stations  was  of  some  importance. 
Setif  is  situated  about  eleven  hundred  metres  above  the  level  of  the  sea,  on  a  buttress 
of  the  southern  slope  of  the  high  mountains  of  the  sea-coast,  the  chain  of  theBabere 
or  the  Bibans.  Towards  the  south  it  overlooks  from  a  height  of  two  or  three  hun- 
dred metres  an  immense  plain,  which  in  its  turn  isboi'dered  at  a  distance  of  35  or  40 
kilometres  (22  to  25  miles)  by  a  chain  of  not  very  high  mountains,  which  cuts  it 
off  from  another  more  extensive  plain,  the  basin  of  the  Hodna.  Beyond  this,  se[)a- 
rated  again  by  an  insignificant  mountainous  elevation,  lies  the  Sahara  and  the  desert 
climate,  which  makes  its  influence  felt  as  far  as  the  plain  of  Setif.  This  vast  heat- 
ing-centre south  of  the  city  frequently  procures  for  the  lattei',  towards  evening,  a 
fresh  current  of  air  from  the  north,  and  in  ordinaiy  times  it  stands  on  the  boundary 
line  where  two  contrary  influences  enter  into  direct  conflict,  the  wind  blowing  fi'om 
the  coast  and  the  high  summits,  and  the  burning  wind  from  the  desei't.  Thus 
Setif  enjoys  a  relative  freshness  on  certain  days  when  the  plain  at  its  feet  is  given 
up  to  the  full  ardor  of  the  sirocco,  and  when  it  even  may  happen  that  the  cloud  of 
dust,  propelled  by  this  wind,  stopping  at  a  distance  of  15  or  20  kilos,  from  the 
town,  screens  the  neighboring  mountains  at  the  very  time  when  the  atmosphere 
remains  quite  clear  about  Setif  and  the  immediate  surroundings. 

The  station  which  I  have  chosen  in  France  i'oi-  my  comparative  observations 
is  also  situated  on  the  side  of  a  slope,  overlooking  the  plain  of  Vic-sur-C^re  (Cantal), 
and  750  metres  above  the  level  of  the  sea.     I  might  have  gone  higher,  but  I  have 

44 


ATMOSPHERIC  ACTINOMETRY. 


45 


alread}'  shown  that  the  difference  in  altitude  is  of  little  importance.  At  all  events 
it  acted  in  the  opposite  sense  to  the  phenomenon  which  I  sought  to  verify.  As  a 
compensation,  the  climate  of  this  station  at  Olmet  is  a  temperate  climate.  The 
place  lies  on  the  line  where  the  culture  of  the  vine  ceases  and  is  in  every  respect 
equal  to  that  of  Fau,  in  the  valley  of  Marmanhac,  and  of  Noalhac,  in  the  valley  of 
Aurillac,  where  I  had  made  my  first  observations. 

Furthermore,  the  procedure  of  M.  Gessard  and  myself  was  the  same;  we 
exposed,  from  8  o'clock  a.m.  till  5  o'clock  p.m.,  vessels  containing  the  same  solution, 
only,  on  account  of  the  high  temperature  of  Setif  during  the  summer  and  of  the  evap- 
oiation  caused  by  it,  we  had  to  pour  into  the  vessels  20  cubic  centimetres  of  oxalic 
acid,  instead  of  10,  and  place  them,  not  upon  wooden  or  stone  suppoi-ts,  but  upon 
the  water  of  a  great  crystallizing  pan.  I  need  not  say  that  at  Olmet  I  followed 
the  same  practice.  It  is  well  known  that  the  degree  of  solar  combustion  depends 
on  the  depth  of  the  liquid,  and  this  is  the  reason  why  the  present  series  of  experi- 
ments is  not  directly  comparable  with  the  preceding  series.  But  it  is  sufficient  for 
us  that  the  expeiiments  made  in  France  and  in  Algeria  should  be  comparable 
between  themselves. 

This  being  granted,  I  subjoin  the  i-esults  obtained  by  M.  Gessard  : 


Date. 

Solar 
Combustion. 

July 

2 

12 

3 

14 

4 

12 

5 

13 

6 

i6 

7 

'7 

8 

62  ' 

9 

18 

1 1 

40 

12 

32 

'3 

22 

i.S 

25 

17 

22 

i8 

22 

19 

24 

20 

22 

21 

23 

Remarks. 


Clouds. 

Stormy  weather.     Sky  covered  at  3  o'clock. 

Fewer  clouds  than  on  preceding  days. 

Clear  weather.     No  dust. 

Cloudy  at  times.     (See  note  below.) 

Tempest.     Dust.     Sun  obscured  after  noon. 

Cloudy  weather. 

Fine  weather. 

Overcast  weather  ;  a  few  drops  of  rain. 

Clouds,  storms,  dust  repeatedly. 

Fine  weather.     Hail-storm  at  7  p.m. 

The  Southern  mountain  cannot  be  seen  at  5  p.m. 

No  clouds.     Very  clear  horizon. 

Overcast  from  2  to  4.30  p.m.     Dust. 

Overcast  after  noon.     Downpour  at  3  p.m. 


In  the  following  experiments,  made  in  August  and  September,  M.  Gessard 
ascei-tained  the  direction  of  the  wind  and  the  temperature,  as  read  on  a  thermometer 


'  One  of  the  vessels  was  found  to  be  submerged  on  this  day,  so  that  the  result  could  not  be 
correctly  ascertained.  The  weather  on  this  day  did  not  essentially  differ  from  that  of  the  preced- 
ing and  of  the  following  days. 


46 


ATMOSPHERIC  ACTINOMETRY 


hanging  in  a  northern  exposure,  against  the  wall  of  a  house  with  lofty  arcades,  and 
consequently  under  a  galleiy  formed  by  them.  In  the  statements  concerning  the 
winds,  the  frequent  violent  changes  of  which  we  spoke  at  the  beginning  will  be 
noticed. 


Temperature. 

Winds  (by 

the  vane 

)at 

Date, 

Solar 

Kemarks 

Combustion. 

M.^\i*tll<&4  a^^t 

Max. 

Min. 

Aver. 

8.30 

11 

3 

5 

August 

i8 

6^ 

Clouds  at   noon.      Overcast 

sky  at  4.     Rain  at  4.30. 

34- 

15-2 

3'-5 

W 

w 

N 

SW 

ti 

19 

3^ 

Quite  a  fine  day.    g.  g.  clouds 

at  2  o'clock. 

30.2 

14- 

27-5 

E 

s 

NNE 

NNE 

a 

20 

3^ 

Sun  rather  overcast   in   the 
morning.      Rain    after    2 

o'clock. 

27.9 

12.5 

25- 

BE 

N 

N 

NE 

<i 

21 

3^ 

Sun    all    day  long   and    sky 

clear. 

28.6 

158 

25-5 

W 

w 

NW 

N 

<t 

22 

8^ 

The  same. 

311 

17-5 

28.5 

W 

w 

N 

NE 

ti 

23 

8^ 

4<                  ti 

32.6 

18.6 

30- 

SW 

w 

S 

SW 

it 

24 

11^ 

((       (( 

33-2 

17.2 

31-5 

SW 

SW 

S 

SW 

(i 

25 

6^ 

((       ti 

31-4 

17- 

19. 

SW 

w 

S 

SW 

« 

26 

8^ 

Clouds  in  the  afternoon. 

32.6 

17.8 

29s 

SW 

SW 

s 

SW 

it 

27 

9^ 

Clouds     afternoon.           Sun 

shines  at  3  p.m. 

31.8 

16.2 

295 

W 

w 

NE 

NE 

it 

28 

8^ 

Sun  all  day  long. 

31-3 

16.3 

28.S 

W 

SW 

NNW 

NNE 

a 

29 

Ti 

Same. 

311 

16.3 

29. 

SW 

SW 

SW 

NE 

a 

30 

li 

ti 

31.6 

18.6 

30- 

SW 

SW 

SW 

NE 

a 

31 

Ti 

it 

32.2 

19.0 

29s 

SE 

SE 

SE 

SSE 

September   8 

9^ 

Overcast.     Much  dust. 

25-4 

17- 

29-5 

E 

s 

SSE 

S 

l4 

19 

10  ^ 

Sun  all  day  long. 

29. 

15-6 

29s 

SE 

s 

SSE 

S 

a 

22 

11^ 

A  few  clouds  all  day  long. 

29. 

14.8 

27- 

W 

NW 

w 

N 

23 
24 

II  i 
II  ^ 

Sun  all  day. 

Sun  rather  obscured,  after- 

29.4 

17.7 

26.5 

W 

NW 

w 

NW 

noon.     Dust. 

30-9 

17.1 

28. 

SW 

SW 

SW 

SW 

i< 

25 

14$^ 

Overcast  sun  all  day.     Dust. 

30.8 

17- 

28. 

s 

SW 

SW 

S 

When  M.  Gessard  sent  me  the  reports  for  August  and  September,  he  added 
what  follows  :  "They  seem  to  me  to  confirm  the  former.  The  combustions  have 
not  been  more  intense  than  in  July.  I  notice,  on  the  other  hand,  that  during  over- 
cast weather  they  have  not  been  inferior  to  those  observed  on  days  of  full  sunshine. 
The  second  fact  which  strikes  me  is  this  :  that  the  quality  of  combustion  should 
be  maintained  with  such  constancy  in  the  two  vessels  under  all  circumstances. 
Thus,  on  the  11th  September  (I  did  not  report  it  in  the  table),  with  a  south  wind 
blowing  all  day,  raising  the  maximum  temperature  to  32°  4',  the  contents  of  both 
vessels  having  evapoi-ated,  although  they  had  been  allowed  to  float  upon  the  water, 
I  had  the  cunosity  to  re-dissolve  the  residual  acid  and  to  test  it :  I  found  8.4  c.  c. 


AND  THE  ACTINIC  CONSTITUTION  OF  THE   ATMOSPHERE. 


47 


and  8.6  c.  c,  which  corresponds  to  combustions  of  36  per  cent  and    35   per  cent, 
respectively. 

It  will  be  seen  from  all  that  has  been  said  so  far,  that  the  daily  actinometric 
combustions  are  quite  as  irregular  in  Algeria  as  in  France,  and  have  only  a  very 
remote  connection  with  the  external  aspect  of  the  sky  and  the  clearness  of  the 
horizon.  1  have  not  been  so  fortunate  as  M.  Gessard  so  far  as  the  weather  was 
concerned.  The  gi'eater  part  of  the  month  of  August  in  the  Cantal  and  a  part 
of  the  month  of  September  we  had  cloudy  or  rainy  periods,  and  during  this  time 
obsei'vations  were  impossible.  Availing  myself  of  the  fact  that  the  method  of 
comparison  which  we  employed  consists  of  a  compaiisou  of  the  finest  days  at  the 
two  stations,  I  shall  here  I'eport  only  the  figures  noted  down  at  Olmet  during  the 
rare  times  of  fine  weather. 


Date. 

Solar 
Combustion. 

Remarks. 

August 

27 

29  ^ 

— ■ — — ■ — ■ 

Fine  day  from  beginning  to  end. 

*i 

28 

42  ^ 

Fine  weather,  but  rather  heavy  and  stormy. 

n 

29 

41  ^ 

A  fine  day,  although  rather  foggy. 

It 

30 

35  ^ 

A  fine  day,  rather  stormy.     Some  clouds. 

a 

31 

41  !« 

A  fine  day.     Some  mist  towards  4  o'clock  in  the  west. 

September   2 

27  ;< 

Clear  in  the  morning  ;  rather  foggy  in  the  evening. 

17 

40^ 

Quite  a  fine  day  in  spite  of  g.  g.  clouds.     N.  E.  wind. 

18 

34  !« 

Cumulus  covering  }  of  sky,  all  day  long. 

19 

52  ^ 

Less  cumulus  than  the  day  before.     Finer  day. 

20 

60^ 

A  very  fine  day. 

21 

52  ^ 

Rather  stormy  day. 

27 

28^ 

A  fine  day  ;  few  clouds. 

29 

23  ^ 

An  indifferent  day. 

Comparing  this  list  with  M.  Gessard's  we  notice  in  the  first  place  a  coincidence 
between  the  days  of  August  27th,  28th,  29th,  30th,  and  31st,  which  were  eithei- 
fine  or  very  fine  at  Olmet  and  at  Setif.  Now  if  we  compai-e  the  results  of  the 
daily  combustion  fi'om  8  a.m.  till  5  p.m.  we  find,  respectively : 

9,    8,    7,    7,  and    7  for  Setif ; 
and  29,  42,  41,  35,  and  31  for  Olmet. 

If  we  in  like  manner  compare,  leaving  out  the  dates,  the  combustion  on  the 
finest  days  in  France  and  in  Algeria,  we  find  again  that  it  decreases  with  the  lati- 
tude. This  is  the  same  conclusion  to  M'^hich  we  were  led,  when  we  compared  the 
observations  made  in  Fi'ance  and  in  Norway,  and  between  the  limits  of  the  two 
stations  at  Helsingfors  (lat.  60°  19'),  and  at  Setif  (lat.  36°  1 1').     This  shows  that  the 


48  ATMOSPHERIC  ACTINOMETRY. 

actinic  effect  of  twilight  during  equal  times  of  insolation  goes  on  diminishing  in 
proportion  as  we  approach  the  equator  and  as  the  mean  temperature  rises. 

It  would  be  interesting  to  ascertain  whether  this  law  continues  into  the 
tropical  regions.  This  is  probable,  but  as  yet  is  only  a  matter  of  conjecture.  I  have 
taken  the  proper  steps  to  begin  observations  on  this  subject  early  next  spring. 

Whatever  this  extension  may  lead  to,  it  is  none  the  less  unexpected  to  find 
that  in  our  temperate  regions,  the  most  densely  populated  of  the  globe,  the  actinic 
effect  of  the  sun  is,  so  to  speak,  in  the  opposite  direction  to  its  calorific  influence. 
The  operation  of  this  law  on  the  flora  cannot  be  doubted.  But  the  future  alone 
can  tell  us  how  this  is  brought  about. 


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